Low bandwidth phy for wlan

ABSTRACT

A method, in a communication system utilizing channels for transmitting first PHY mode data units, includes generating first and second data units conforming to first and second PHY modes, respectively, causing the first data unit to be transmitted via a channel, determining a frequency band for transmitting the second data unit, and causing the second data unit to be transmitted via the frequency band. Generating the first and second data units includes generating first and second series of OFDM symbols, respectively. At least a portion of the second OFDM symbols includes more upper-edge than lower-edge guard tones, or vice versa. The frequency band has a bandwidth equal to the channel bandwidth divided by n≧2, and either a lowest or highest sub-band of one or more channels is excluded from the frequency band when the second OFDM symbols include more upper-edge or more lower-edge guard tones, respectively.

CROSS-REFERENCES TO RELATED APPLICATIONS

This disclosure claims the benefit of the following U.S. ProvisionalPatent Applications:

-   U.S. Provisional Patent Application No. 61/497,274, entitled “11ah    OFD Bandwidth PHY,” filed on Jun. 15, 2011:-   U.S. Provisional Patent Application No. 61/513,452, entitled “11ah    OFDM Low Bandwidth PHY,” filed on Jul. 29, 2011;-   U.S. Provisional Patent Application No. 61/514,164, entitled “11ah    OFDM Low Bandwidth PHY.” filed on Aug. 2, 2011;-   U.S. Provisional Patent Application No. 61/523,014, entitled “11ah    OFDM Low Bandwidth PHY,” filed on Aug. 12, 2011;-   U.S. Provisional Patent Application No. 61/523,799, “11 ah OFDM Low    Bandwidth PHY,” filed on Aug. 15, 2011;-   U.S. Provisional Patent Application No. 61/524,231, entitled “11ah    OFDM Low Bandwidth PHY,” filed on Aug. 16, 2011;-   U.S. Provisional Patent Application No. 61/531,548, entitled “11ah    OFDM Low Bandwidth PHY,” filed on Sep. 6, 2011;-   U.S. Provisional Patent Application No. 61/534,641, entitled “11ah    OFDM Low Bandwidth PHY.” filed on Sep. 14, 2011;-   U.S. Provisional Patent Application No. 61/537,169, entitled “11 ah    OFDM Low Bandwidth PHY,” filed on Sep. 21, 2011;-   U.S. Provisional Patent Application No. 61/550,321, entitled “11ah    OFDM Low Bandwidth PHY,” filed on Oct. 21, 2011; and-   U.S. Provisional Patent Application No. 61/552,631, entitled “11ah    OFDM Low Bandwidth PHY,” filed on Oct. 28, 2011.    The disclosures of all of the above-referenced patent applications    are hereby incorporated by reference herein in their entireties.

The present application is related to U.S. patent application Ser. Nos.______ (Attorney Docket No. MP4182) and (Attorney Docket No. MP4182.C1),both entitled “Low Bandwidth PHY for WLAN,” both filed on the same dayas the present application, and both hereby incorporated by referenceherein in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication networks and,more particularly, to long range low power wireless local area networks.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

When operating in an infrastructure mode, wireless local area networks(WLANs) typically include an access point (AP) and one or more clientstations. WLANs have evolved rapidly over the past decade. Developmentof WLAN standards such as the Institute for Electrical and ElectronicsEngineers (IEEE) 802.11a, 802.11b, 802.11g, and 802.11n Standards hasimproved single-user peak data throughput. For example, the IEEE 802.11bStandard specifies a single-user peak throughput of 11 megabits persecond (Mbps), the IEEE 802.11a and 802.11g Standards specify asingle-user peak throughput of 54 Mbps, the IEEE 802.11n Standardspecifies a single-user peak throughput of 600 Mbps, and the IEEE802.11ac Standard specifies a single-user peak throughput in thegigabits per second (Gbps) range.

Work has begun on a two new standards, IEEE 802.11ah and IEEE 802.11af,each of which will specify wireless network operation in sub-1 GHzfrequencies. Low frequency communication channels are generallycharacterized by better propagation qualities and extended propagationranges compared to transmission at higher frequencies. In the past,sub-1 GHz ranges have not been utilized for wireless communicationnetworks because such frequencies were reserved for other applications(e.g., licensed TV frequency bands, radio frequency band, etc.). Thereare few frequency bands in the sub-1 GHz range that remain unlicensed,with different specific unlicensed frequencies in different geographicalregions. The IEEE 802.11ah Standard will specify wireless operation inavailable unlicensed sub-1 GHz frequency bands. The IEEE 802.11afStandard will specify wireless operation in TV White Space (TVWS), i.e.,unused TV channels in sub-1 GHz frequency bands.

SUMMARY

In one embodiment, a method, in a communication system, of generatingand causing to be transmitted data units conforming to a first physicallayer (PHY) mode and data units conforming to a second PHY modedifferent than the first PHY mode, wherein the communication systemutilizes a plurality of channels for transmitting data units conformingto the first PHY mode, and wherein each channel of the plurality ofchannels has a first bandwidth, includes generating a first data unitconforming to the first PHY mode. Generating the first data unitincludes generating a first series of orthogonal frequency divisionmultiplexing (OFDM) symbols. The method also includes causing the firstdata unit to be transmitted via a channel of the plurality of channels,and generating a second data unit conforming to the second PHY mode.Generating the second data unit includes generating a second series ofOFDM symbols. At least a portion of the second series of OFDM symbolsincludes one of (i) more upper-edge guard tones than lower-edge guardtones, or (ii) more lower-edge guard tones than upper-edge guard tones.The method also includes determining a frequency band for transmittingthe second data unit. The frequency band has a second bandwidth equal tothe first bandwidth divided by an integer n, where n≧2. Determining thefrequency band for transmitting the second data unit includes excludingone of (i) a lowest sub-band of each of one or more channels in theplurality of channels when the portion of the second series of OFDMsymbols includes more upper-edge guard tones than lower-edge guard tonesor (ii) a highest sub-band of each of the one or more channels in theplurality of channels when the portion of the second series of OFDMsymbols includes more lower-edge guard tones than upper-edge guardtones. Each sub-band of each channel in the plurality of channels hasthe second bandwidth. The method also includes causing the second dataunit to be transmitted via the determined frequency band.

In another embodiment an apparatus includes a network interfaceconfigured to generate a first data unit conforming to a first PHY modeat least in part by generating a first series of OFDM symbols, cause thefirst data unit to be transmitted via a channel of a plurality ofchannels each having a first bandwidth, and generate a second data unitconforming to a second PHY mode different than the first PHY mode atleast in part by generating a second series of OFDM symbols. At least aportion of the second series of OFDM symbols includes one of (i) moreupper-edge guard tones than lower-edge guard tones, or (ii) morelower-edge guard tones than upper-edge guard tones. The networkinterface is also configured to determine a frequency band fortransmitting the second data unit. The frequency band has a secondbandwidth equal to the first bandwidth divided by an integer n, wheren≧2. The network interface is configured to determine the frequency bandfor transmitting the second data unit at least in part by excluding oneof (i) a lowest sub-band of each of one or more channels in theplurality of channels when the portion of the second series of OFDMsymbols includes more upper-edge guard tones than lower-edge guard tonesor (ii) a highest sub-band of each of the one or more channels in theplurality of channels when the portion of the second series of OFDMsymbols includes more lower-edge guard tones than upper-edge guardtones. Each sub-band of each channel in the plurality of channels hasthe second bandwidth. The network interface is also configured to causethe second data unit to be transmitted via the determined frequencyband.

In another embodiment, a method, in a communication system, ofgenerating and causing to be transmitted data units conforming to afirst PHY mode and data units conforming to a second PHY mode differentthan the first PHY mode, wherein the communication system utilizes aplurality of channels for transmitting data units conforming to thefirst PHY mode, and wherein each channel of the plurality of channelshas a first bandwidth, includes generating a first data unit conformingto the first PHY mode. Generating the first data unit includesgenerating a first series of OFDM symbols utilizing a clock rate. Themethod also includes causing the first data unit to be transmitted via achannel of the plurality of channels, and generating a second data unitconforming to the second PHY mode. Generating the second data unitincludes generating a second series of OFDM symbols utilizing the clockrate. At least a data portion of the second series of OFDM symbolsincludes more lower-edge guard tones than upper-edge guard tones. Themethod also includes determining a frequency band for transmitting thesecond data unit. The frequency band has a second bandwidth equal tohalf the first bandwidth. Determining the frequency band fortransmitting the second data unit includes excluding an upper sidebandof each of one or more channels in the plurality of channels. The methodalso includes causing the second data unit to be transmitted via thedetermined frequency band.

In another embodiment, an apparatus includes a network interfaceconfigured to generate a first data unit conforming to a first PHY modeat least in part by generating a first series of OFDM symbols utilizinga clock rate, cause the first data unit to be transmitted via a channelof a plurality of channels each having a first bandwidth, and generate asecond data unit conforming to a second PHY mode different than thefirst PHY mode at least in part by generating a second series of OFDMsymbols utilizing the clock rate. At least a data portion of the secondseries of OFDM symbols includes more lower-edge guard tones thanupper-edge guard tones. The network interface is also configured todetermine a frequency band for transmitting the second data unit. Thefrequency band has a second bandwidth equal to half the first bandwidth.The network interface is configured to determine the frequency band fortransmitting the second data unit at least in part by excluding an uppersideband of each of one or more channels in the plurality of channels.The network interface is also configured to cause the second data unitto be transmitted via the determined frequency band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example wireless local area network(WLAN), according to an embodiment.

FIG. 2 is a block diagram of a transmit portion of an example physicallayer (PHY) processing unit for generating normal mode data units,according to an embodiment.

FIG. 3 is a block diagram of a transmit portion of an example PHYprocessing unit for generating low bandwidth mode data units, accordingto an embodiment.

FIG. 4 is a block diagram of a transmit portion of another example PHYprocessing unit for generating low bandwidth mode data units, accordingto an embodiment.

FIG. 5 is a block diagram of a transmit portion of another example PHYprocessing unit for generating low bandwidth mode data units, accordingto an embodiment.

FIG. 6 is a flow diagram of an example method for generating first andsecond data units corresponding to first and second PHY modes,respectively, according to an embodiment.

FIGS. 7A and 7B are diagrams of example tone maps corresponding to lowbandwidth mode data units, according to two embodiments.

FIG. 8 is a diagram of example normal mode data units having differentbandwidths, according to an embodiment.

FIG. 9 is a diagram of a preamble of an example low bandwidth mode dataunit, according to an embodiment.

FIG. 10 is a diagram of an example short training field (STF) of anormal mode data unit and an example STF of a low bandwidth mode dataunit, according to an embodiment.

FIG. 11 is a diagram of another example STF of a normal mode data unitand another example STF of a low bandwidth mode data unit, according toan embodiment.

FIG. 12 is a diagram of an example second preamble portion of a normalmode data unit and an example second preamble portion of a low bandwidthmode data unit, according to an embodiment.

FIG. 13 is a diagram illustrating example modulation techniques used tomodulate symbols within fields of a preamble, according to anembodiment.

FIG. 14 is a flow diagram of an example method for generating a firstpreamble for a first data unit corresponding to a first PHY mode and asecond preamble for a second data unit corresponding to a second PHYmode, according to an embodiment.

FIG. 15 is a diagram of another example second preamble portion of anormal mode data unit and another example second preamble portion of alow bandwidth mode data unit, according to an embodiment.

FIG. 16 is a diagram of example second preamble portions of single-userand multi-user normal mode data units, according to an embodiment.

FIG. 17 is a flow diagram of another example method for generating afirst preamble for a first data unit corresponding to a first PHY modeand a second preamble for a second data unit corresponding to a secondPHY mode, according to an embodiment.

FIG. 18 is a diagram of an example placement of a frequency band used totransmit a low bandwidth mode data unit within a communication channelused to transmit a normal mode data unit, according to an embodiment.

FIG. 19 is a diagram of another example placement of a frequency bandused to transmit a low bandwidth mode data unit within a communicationchannel used to transmit a normal mode data unit, according to anembodiment.

FIG. 20 is a diagram of another example placement of a frequency bandused to transmit a low bandwidth mode data unit within a communicationchannel used to transmit a normal mode data unit, according to anembodiment.

FIG. 21 is a diagram of another example placement of a frequency bandused to transmit a low bandwidth mode data unit within a communicationchannel used to transmit a normal mode data unit, according to anembodiment.

FIGS. 22A, 22B, and 22C are diagrams of example regular, reversed, andshifted tone maps, respectively, corresponding to low bandwidth modedata units, according to an embodiment.

FIGS. 23A and 23B are diagrams of example regular and shifted tone maps,respectively, corresponding to low bandwidth mode data units, accordingto an embodiment.

FIG. 24 is a flow diagram of an example method for generating andcausing to be transmitted first and second data units conforming tofirst and second PHY modes, respectively, according to an embodiment.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

In embodiments described below, a wireless network device such as anaccess point (AP) of a wireless local area network (WLAN) transmits datastreams to one or more client stations. The AP is configured to operatewith client stations according to at least a first communicationprotocol. The first communication protocol defines operation in a sub-1GHz frequency range, and is typically used for applications requiringlong range wireless communication with relatively low data rates. Thefirst communication protocol (e.g., IEEE 802.11af or IEEE 802.11ah) isreferred to herein as a “long range” communication protocol. In someembodiments, the AP is also configured to communicate with clientstations according to one or more other communication protocols whichdefine operation in generally higher frequency ranges and are typicallyused for closer-range communications with higher data rates. The higherfrequency communication protocols (e.g., IEEE 802.11a, IEEE 802.11n,and/or IEEE 802.11ac) are collectively referred to herein as “shortrange” communication protocols. In some embodiments, physical layer(PHY) data units conforming to the long range communication protocol(“long range data units”) are the same as or similar to data unitsconforming to a short range communication protocol (“short range dataunits”), but are generated using a lower clock rate. To this end, in anembodiment, the AP operates at a clock rate suitable for short rangeoperation, and down-clocking is used to generate a clock to be used forthe sub-1 GHz operation. As a result, in this embodiment, a long rangedata unit maintains the physical layer format of a short range dataunit, but is transmitted over a longer period of time.

In addition to this “normal mode” specified by the long rangecommunication protocol, in some embodiments, the long rangecommunication protocol also specifies a “low bandwidth mode” with areduced bandwidth and data rate compared to the lowest bandwidth anddata rate specified for the normal mode. Because of the lower data rate,the low bandwidth mode further extends communication range and generallyimproves receiver sensitivity. Data units corresponding to the lowbandwidth mode are generated utilizing the same clock rate as data unitscorresponding to the normal mode (e.g., are down-clocked by the sameratio used for normal mode data units). For example, orthogonalfrequency division multiplexing (OFDM) symbols of normal mode and lowbandwidth mode data units both have the same subcarrier/tone spacing andOFDM symbol duration, in an embodiment. In some embodiments, the normalmode and/or low bandwidth mode include multiple PHY sub-modes. In oneembodiment, for example, the normal mode includes a first sub-modecorresponding to 2 MHz data units, a second sub-mode corresponding to 4MHz data units, etc., and the low bandwidth mode corresponds to only 1MHz data units. In another embodiment, the low bandwidth mode likewiseincludes multiple sub-modes corresponding to data units having differentbandwidths (e.g. 1 MHz, 0.5 MHz, etc.).

The function of the low bandwidth mode may depend on the region in whichthe mode is utilized. For example, in one embodiment of an IEEE 802.11ahsystem in the United States, where a relatively large amount of spectrumis available in sub-1 GHz frequencies, normal mode communicationsutilize channels having at least a minimum bandwidth (e.g., 2 MHz, or2.5 MHz, etc.), and the low bandwidth mode serves as a “control mode”having an even smaller bandwidth (e.g., 1 MHz, or 1.25 MHz, etc.). In anembodiment, the AP uses the control mode for signal beacon orassociation procedures, and/or for transmit beamforming trainingoperations, for example. As another example, in one embodiment of acommunication system in which less spectrum is available in sub-1 GHzfrequencies (e.g., Europe or Japan), the low bandwidth mode serves as anextension of the normal mode rather than a control mode.

FIG. 1 is a block diagram of an example WLAN 10 including an AP 14,according to an embodiment. The AP 14 includes a host processor 15coupled to a network interface 16. The network interface 16 includes amedium access control (MAC) processing unit 18 and a physical layer(PHY) processing unit 20. The PHY processing unit 20 includes aplurality of transceivers 21, and the transceivers 21 are coupled to aplurality of antennas 24. Although three transceivers 21 and threeantennas 24 are illustrated in FIG. 1, the AP 14 can include differentnumbers (e.g., 1, 2. 4, 5, etc.) of transceivers 21 and antennas 24 inother embodiments.

The WLAN 10 further includes a plurality of client stations 25. Althoughfour client stations 25 are illustrated in FIG. 1, the WLAN 10 caninclude different numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations25 in various scenarios and embodiments. At least one of the clientstations 25 (e.g., client station 25-1) is configured to operate atleast according to the long range communication protocol. In someembodiments, at least one of the client stations 25 (e.g., clientstation 25-4) is a short range client station that is configured tooperate at least according to one or more of the short rangecommunication protocols.

The client station 25-1 includes a host processor 26 coupled to anetwork interface 27. The network interface 27 includes a MAC processingunit 28 and a PHY processing unit 29. The PHY processing unit 29includes a plurality of transceivers 30, and the transceivers 30 arecoupled to a plurality of antennas 34. Although three transceivers 30and three antennas 34 are illustrated in FIG. 1, the client station 25-1can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers30 and antennas 34 in other embodiments.

In some embodiments, one, some, or all of the client stations 25-2,25-3, and 25-4 has/have a structure the same as or similar to the clientstation 25-1. In these embodiments, the client stations 25 structuredthe same as or similar to the client station 25-1 have the same or adifferent number of transceivers and antennas. For example, the clientstation 25-2 has only two transceivers and two antennas, according to anembodiment.

In various embodiments, the PHY processing unit 20 of the AP 14 isconfigured to generate data units conforming to the long rangecommunication protocol and having formats described hereinafter. Thetransceiver(s) 21 is/are configured to transmit the generated data unitsvia the antenna(s) 24. Similarly, the transceiver(s) 24 is/areconfigured to receive the data units via the antenna(s) 24. The PHYprocessing unit 20 of the AP 14 is also configured to process receiveddata units conforming to the long range communication protocol andhaving formats described hereinafter, according to various embodiments.

In various embodiments, the PHY processing unit 29 of the client device25-1 is configured to generate data units conforming to the long rangecommunication protocol and having formats described hereinafter. Thetransceiver(s) 30 is/are configured to transmit the generated data unitsvia the antenna(s) 34. Similarly, the transceiver(s) 30 is/areconfigured to receive data units via the antenna(s) 34. The PHYprocessing unit 29 of the client device 25-1 is also configured toprocess received data units conforming to the long range communicationprotocol and having formats described hereinafter, according to variousembodiments.

In some embodiments, the AP 14 is configured to operate in dual bandconfigurations. In such embodiments, the AP 14 is able to switch betweenshort range and long range modes of operation. According to one suchembodiment, when operating in short range mode, the AP 14 transmits andreceives data units that conform to one or more of the short rangecommunication protocols. When operating in a long range mode, the AP 14transmits and receives data units that conform to the long rangecommunication protocol. Similarly, the client station 25-1 is capable ofdual frequency band operation, according to some embodiments. In theseembodiments, the client station 25-1 is able to switch between shortrange and long range modes of operation. In other embodiments, the AP 14and/or the client station 25-1 is dual band device that is able toswitch between different low frequency bands defined for long rangeoperations by the long range communication protocol. In yet anotherembodiment, the AP 14 and/or the client station 25-1 is a single banddevice configured to operate in only one long range frequency band.

In still other embodiments, the client station 25-1 is a dual modedevice capable of operating in different regions with differentcorresponding PHY modes. For example, in one such embodiment, the clientstation 25-1 is configured to utilize the normal mode PHY when operatingin a first region, and to utilize the low bandwidth mode PHY whenoperating in a second region (e.g., a region with less availablespectrum). In an embodiment, the client station 25-1 can switch betweennormal and low bandwidth modes in the different regions by switchingbetween low bandwidth mode and normal mode baseband signal processing ofthe transmitter and receiver, and switching digital and analog filtersto meet the requirements applicable to each mode (e.g., spectral maskrequirements at the transmitter, adjacent channel interferencerequirements at the receiver, etc.). Hardware settings such as clockrate, however, are unchanged when switching between low bandwidth modeand normal mode, in an embodiment.

In one example embodiment, client station 25-1 is a dual mode devicethat utilizes a normal mode PHY in the U.S. (e.g., for 2 MHz and widerchannels) and a low bandwidth mode in Europe and/or Japan (e.g., for 1MHz channels). The same clock rate is used globally, in this embodiment,with different inverse discrete Fourier transform (IDFT) sizes beingutilized to generate signals of different bandwidths (e.g., a 64-pointor larger IDFT for the 2 MHz or wider bandwidth U.S. channels, and a32-point IDFT for the 1 MHz Europe/Japan channels). In some of theseembodiments, the low bandwidth mode is also used for control PHY in theU.S.

In another example embodiment, client station 25-1 is a dual mode devicethat in the U.S. utilizes a normal mode PHY (e.g., for 2 MHz and widerchannels) and a low bandwidth mode PHY (e.g. for control mode signalshaving a 1 MHz bandwidth), and in Europe and/or Japan utilizes only thelow bandwidth mode PHY (e.g., for 1 MHz channels). The same clock rateis used globally, in this embodiment, with different IDFT sizes beingused to generate signals of different bandwidths (e.g., a 64-point orlarger IDFT for the 2 MHz or wider bandwidth U.S. channels, and a32-point IDFT for both the 1 MHz U.S. control mode signals and the 1 MHzEurope/Japan channels).

In some embodiments, devices such as client station 25-1 use the samesize IDFT (at a constant clock rate) whether generating asmallest-bandwidth normal mode data unit or a low bandwidth mode dataunit. For example, in one embodiment, a 64-point IDFT is used togenerate both a 2 MHz normal mode data unit and a 1 MHz low bandwidthmode data unit, with the appropriate tones being zeroed out in thelatter case. In some scenarios for these embodiments, filters need notbe changed on the fly when changing between PHY modes, while stillmeeting the spectral mask requirements for the wider (e.g., 2 MHz)channel. In other scenarios, a transmitted low bandwidth mode signal isrequired to meet a tighter, lower bandwidth spectral mask even iftransmitted using an IDFT size corresponding to a wider bandwidth.

FIG. 2 is a block diagram of a transmit portion of an example PHYprocessing unit 100 for generating normal mode data units, according toan embodiment. Referring to FIG. 1, the PHY processing unit 20 of AP 14and the PHY processing unit 29 of client station 25-1 are each similarto or the same as PHY processing unit 100, in one embodiment. The PHYprocessing unit 100 includes a scrambler 102 that generally scrambles aninformation bit stream to reduce occurrences of long sequences of onesor zeros, according to an embodiment. An encoder parser 104 is coupledto the scrambler 102. The encoder parser 208 demultiplexes theinformation bit stream into one or more encoder input streamscorresponding to one or more FEC encoders 106.

While two FEC encoders 106 are shown in FIG. 2, different numbers of FECencoders are included, and/or different numbers of FEC encoders operatein parallel, in various other embodiments and/or scenarios. For example,according to one embodiment, the PHY processing unit 100 includes fourFEC encoders 106, and one, two, three, or four of the FEC encoders 106operate simultaneously depending on the particular modulation and codingscheme (MCS), bandwidth, and number of spatial streams. Each FEC encoder106 encodes the corresponding input stream to generate a correspondingencoded stream. In one embodiment, each FEC encoder 106 includes abinary convolutional coder (BCC). In another embodiment, each FEC 106encoder includes a BCC followed by a puncturing block. In anotherembodiment, each FEC encoder 106 includes a low density parity check(LDPC) encoder.

A stream parser 108 parses the one or more encoded streams into one ormore spatial streams (e.g., four streams in the example PHY processingunit 100 shown in FIG. 2) for separate interleaving and mapping intoconstellation points/symbols. In one embodiment, the stream parser 108operates according to the IEEE 802.11n communication protocol, such thatthe following equation is satisfied:

$\begin{matrix}{s = {\max \left\{ {1,\frac{N_{BPSCS}}{2}} \right\}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where s is the number of coded bits assigned to a single axis in aconstellation point for each of N_(SS) spatial streams, and whereN_(BPSCS) is the number of bits per subcarrier. For each FEC encoder 106(whether BCC or LDPC), consecutive blocks of s coded bits are assignedto different spatial streams in a round robin fashion, in an embodiment.In some embodiments where the set of FEC encoders 106 includes two ormore BCC encoders, the outputs of the individual FEC encoders 106 areused in an alternating fashion for each round-robin cycle, i.e.,initially S bits from the first FEC encoder 106 are fed into N_(SS)spatial streams, then S bits from the second FEC encoder 106 are fedinto the N_(SS) spatial streams, and so on, where:

S=N _(SS) ×s  Equation 2

Corresponding to each of the N_(SS) spatial streams, an interleaver 110interleaves bits of the spatial stream (i.e., changes the order of thebits) to prevent long sequences of adjacent noisy bits from entering adecoder at the receiver. More specifically, the interleaver 110 mapsadjacent coded bits onto non-adjacent locations in the frequency domainor in the time domain. The interleaver 110 operates according to theIEEE 802.11n communication protocol (i.e., two frequency permutations ineach data stream, and a third permutation to cyclically shift bitsdifferently on different streams), in an embodiment, with the exceptionthat the parameters N_(col), N_(row), and N_(rot) (i.e., number ofcolumns, number of rows, and frequency rotation parameter, respectively)are suitable values based on the bandwidth of the long range, normalmode data units.

Also corresponding to each spatial stream, a constellation mapper 112maps an interleaved sequence of bits to constellation pointscorresponding to different subcarriers/tones of an OFDM symbol. Morespecifically, for each spatial stream, the constellation mapper 112translates every bit sequence of length log₂(M) into one of Mconstellation points, in an embodiment. The constellation mapper 112handles different numbers of constellation points depending on the MCSbeing utilized. In an embodiment, the constellation mapper 112 is aquadrature amplitude modulation (QAM) mapper that handles M=2, 4, 16,64, 256, and 1024. In other embodiments, the constellation mapper 112handles different modulation schemes corresponding to M equalingdifferent subsets of at least two values from the set {2, 4, 16, 64,256, 1024}.

In an embodiment, a space-time block coding (STBC) unit 114 receives theconstellation points corresponding to the one or more spatial streamsand spreads the spatial streams to a number (N_(STS)) of space-timestreams. In some embodiments, the STBC unit 114 is omitted. Cyclic shiftdiversity (CSD) units 116 are coupled to the STBC unit 114. The CSDunits 116 insert cyclic shifts into all but one of the space-timestreams (if more than one space-time stream) to prevent unintentionalbeamforming. For ease of explanation, the inputs to the CSD units 116are referred to as space-time streams even in embodiments in which theSTBC unit 114 is omitted.

A spatial mapping unit 120 maps the N_(STS) space-time streams to N_(TX)transmit chains. In various embodiments, spatial mapping includes one ormore of: 1) direct mapping, in which constellation points from eachspace-time stream are mapped directly onto transmit chains (i.e.,one-to-one mapping); 2) spatial expansion, in which vectors ofconstellation points from all space-time streams are expanded via matrixmultiplication to produce inputs to the transmit chains; and 3)beamforming, in which each vector of constellation points from all ofthe space-time streams is multiplied by a matrix of steering vectors toproduce inputs to the transmit chains. Each output of the spatialmapping unit 120 corresponds to a transmit chain, and each output of thespatial mapping unit 120 is operated on by an IDFT calculation unit 122(e.g., an inverse fast Fourier transform (IFFT) calculation unit) thatconverts a block of constellation points to a time-domain signal.Outputs of the IDFT units 122 are provided to GI insertion and windowingunits 124 that prepend to OFDM symbols, a guard interval (GI) portion,which is a circular extension of an OFDM symbol in an embodiment, andsmooth the edges of OFDM symbols to increase spectral delay. Outputs ofthe GI insertion and windowing units 124 are provided to analog andradio frequency (RF) units 126 that convert the signals to analogsignals and upconvert the signals to RF frequencies for transmission.The signals are transmitted in a 2 MHz, a 4 MHz, an 8 MHz, or a 16 MHzbandwidth channel (e.g., corresponding to a 64-, 128-, 256-, or512-point IDFT at unit 122, respectively, and utilizing a clock ratethat is constant regardless of IDFT size), in various embodiments and/orscenarios. In other embodiments, other suitable channel bandwidths(and/or IDFT sizes) are utilized. Long range data units corresponding tothe normal mode are discussed in more detail in U.S. patent applicationSer. No. 13/359,336, filed on Jan. 6, 2012 and entitled “Physical LayerFrame Format for Long Range WLAN,” which is hereby incorporated byreference herein in its entirety.

Low bandwidth mode communications are generally more robust than normalmode communications, having a sensitivity gain that supports extendedrange communications. For example, in an embodiment in which the normalmode utilizes a 64-point IDFT (e.g., for a 2 MHz bandwidth signal) togenerate normal mode data units, and in which the low bandwidth modeutilizes a 32-point IDFT (e.g., for a 1 MHz bandwidth signal) togenerate low bandwidth mode data units, the low bandwidth mode providesapproximately a 3 dB sensitivity gain. As another example, in anembodiment in which the normal mode utilizes a 64-point IDFT (e.g., fora 2 MHz bandwidth signal) to generate normal mode data units, and inwhich the low bandwidth mode utilizes a 16-point IDFT (e.g., for a 0.5MHz bandwidth signal) to generate low bandwidth mode data units, the lowbandwidth mode provides approximately a 6 dB sensitivity gain. Moreover,in some embodiments, the low bandwidth mode introduces redundancy orrepetition of bits into at least some fields of the data unit to furtherreduce the data rate. For example, in various embodiments and/orscenarios, the low bandwidth mode introduces redundancy into the dataportion and/or the signal field of a low bandwidth mode data unitaccording to one or more repetition and coding schemes described below.In an embodiment where the low bandwidth mode includes a 2× repetitionof bits, for example, a further 3 dB sensitivity gain may be obtained.Still further, in some embodiments, the low bandwidth mode improvessensitivity by generating OFDM symbols in accordance with the lowestdata rate MCS of the normal mode, or in accordance with an MCS lowerthan the lowest data rate MCS of the normal mode. As an example, in anembodiment, data units in normal mode are generated according to aparticular MCS selected from a set of MCSs, such as MCS0 (binary phaseshift keying (BPSK) modulation and coding rate of 1/2) to MCS9(quadrature amplitude modulation (QAM) and coding rate of 5/6), withhigher order MCSs corresponding to higher data rates. In one suchembodiment, the low bandwidth mode data units are generated usingmodulation and coding as defined by MCS0. In an alternative embodiment,MCS0 is reserved for low bandwidth mode data units only, and cannot beused for normal mode data units.

FIGS. 3-5 are block diagrams of transmit portions of example PHYprocessing units for generating low bandwidth mode data units, accordingto various embodiments. Referring to FIG. 1, the PHY processing unit 20of AP 14 and the PHY processing unit 29 of client station 25-1 are eachsimilar to or the same as any one of the PHY processing units shown inFIGS. 3-5, in various embodiments. In some embodiments, the PHYprocessing units of FIGS. 3-5 correspond to the same hardware as the PHYprocessing unit 100 of FIG. 2, but with different signal processingoperations being utilized within the hardware depending on whethernormal mode or low bandwidth mode data units are being generated.

The PHY processing unit 150 of FIG. 3 includes a scrambler 152 which issimilar to the scrambler 102 of FIG. 2, in an embodiment. The scrambler152 is coupled to one or more FEC encoders 154, which in an embodimentis/are similar to the FEC encoder 106 of FIG. 2. In an embodiment wherethe PHY processing unit 150 includes two or more FEC encoders 154, anencoder parser (not shown) similar to encoder parser 104 of FIG. 2 iscoupled between the scrambler 152 and FEC encoders 154.

A stream parser 158 is coupled to the output(s) of the FEC encoder(s)154. The stream parser 158 is similar to the stream parser 108 of FIG. 2(e.g., Equations 1 and 2, above, are satisfied), in an embodiment, withthe exception that the relevant parameters for Equations 1 and 2 above(e.g., N_(BPSCS) and N_(SS)) match the low bandwidth mode systemparameters (e.g., N_(SS)=1 if only one spatial stream is permitted forlow bandwidth mode data units). The stream parser 158 is coupled to theinterleavers 160. The interleavers 160 are similar to interleavers 110of FIG. 2, in an embodiment, with the exception that the parametersN_(col), N_(row), and N_(rot) are suitable values based on the bandwidthof the low bandwidth data units. For example, in various embodiments inwhich the lowest bandwidth normal mode data units are 2 MHz data unitsgenerated using 64-point IDFTs, and in which the low bandwidth mode dataunits are 1 MHz data units generated using 32-point IDFTs and having 24OFDM data tones, one of the following three options is implemented:

1) N _(col)=12,N _(row)=2×N _(BPSCS)  Equation 3

2) N _(col)=8,N _(row)=3×N _(BPSCS)  Equation 4

3) N _(col)=6,N _(row)=4×N _(BPSCS)  Equation 5

and N_(rot) is one of {2, 3, 4, 5, 6, 7, 8}. For example, in oneparticular embodiment, Equation 4 is satisfied and N_(rot)=2. As anotherexample, in various embodiments in which the lowest bandwidth normalmode data units are 2 MHz data units generated using 64-point IDFTs, andin which the low bandwidth mode data units are 0.5 MHz data units thatare generated using 16-point IDFTs and have 12 OFDM data tones, one ofthe following two options is implemented:

1) N _(col)=6,N _(row)=2×N _(BPSCS)  Equation 6

2) N_(col)=4,N _(row)=3×N _(BPSCS)  Equation 7

and N_(rot) is one of [2, 3, 4, 5].

Corresponding to each spatial stream, a constellation mapper 162 maps aninterleaved sequence of bits to constellation points corresponding todifferent subcarriers/tones of an OFDM symbol. The constellation mappers162 are similar to constellation mappers 112 of FIG. 2, in anembodiment.

In addition to or instead of, any MCS restrictions described above(e.g., low bandwidth mode data units only being permitted to use alowest MCS, etc.), in various embodiments, the allowed MCSs for lowbandwidth mode data units are MCSs that satisfy the following equations:

N _(CBPS) /N _(ES) =m  Equation 8

N _(DBPS) /N _(ES) =n  Equation 9

mod(N _(CBPS) /N _(ES) ,D _(R))=0  Equation 10

R=N _(R) /D _(R)  Equation 11

where N_(CBPS) is the number of coded bits per symbol, N_(DBPS) is thenumber of uncoded bits per symbol, N_(ES) is the number of BCC encoders,m and n are integers. R is the coding rate, and D_(R) is the denominatorof the coding rate (i.e., D_(R)=2 if R=1/2, D_(R)=3 if R=2/3, D_(R)=4 ifR=3/4, and D_(R)=5 if R=5/6). In an embodiment, N_(ES) always equals onefor low bandwidth mode data units (i.e., one spatial stream and one BCCencoder, are used in low bandwidth mode). In other embodiments. N_(ES)is a suitable number greater than one for low bandwidth mode data units.

In an embodiment, an STBC unit 164 (e.g., similar to STBC unit 114 ofFIG. 2) receives the constellation points corresponding to the one ormore spatial streams and spreads the spatial streams to a number ofspace-time streams. A plurality of CSD units 166 (e.g., similar to CSDunits 116 of FIG. 2) are coupled to the STBC unit 164, which in turn iscoupled to a spatial mapping unit 170 (e.g., similar to the spatialmapping unit 120 of FIG. 2). Each output of the spatial mapping unit 170corresponds to a transmit chain, and each output of the spatial mappingunit 120 is operated on by an IDFT unit 172. The IDFT units 172 aresimilar to the IDFT units 122 of FIG. 2 and use the same clock rate asthe IDFT units 122, in an embodiment, but use a smaller size IDFT thanany normal mode data units. For example, in one embodiment where normalmode data units are generated using 64-point or larger IDFTs, lowbandwidth mode data units are generated using 32-point IDFTs. In analternative embodiment in which normal mode data units are generatedusing 64-point or larger IDFTs, low bandwidth mode data units aregenerated using 16-point IDFTs. In another alternative embodiment inwhich normal mode data units are generated using 64-point or largerIDFTs, low bandwidth mode data units are generated using either a16-point or 32-point IDFT depending on which of two PHY sub-modes withina low bandwidth mode is selected.

Outputs of the IDFT units 172 are provided to GI insertion and windowingunits 174 (e.g., similar to GI insertion and windowing units 124 of FIG.2), and outputs of the GI insertion and windowing units 172 are providedto analog and RF units 176 (e.g., similar to analog and RF units 126 ofFIG. 2). In one embodiment, the generated low bandwidth mode data unitsare then transmitted in a low bandwidth mode frequency band. In oneembodiment in which normal mode transmissions utilize 2 MHz and greaterbandwidth (e.g., 4 MHz, 8 MHz, etc.) channels, the frequency band forlow bandwidth mode transmissions is 1 MHz. In other such embodiments,0.5 MHz or another suitable bandwidth less than the minimum normal modechannel bandwidth is utilized.

While the example PHY processing unit 150 of FIG. 3 includes multiplespatial streams (one for each interleaver 160 and constellation mapper162), the low bandwidth mode utilizes only a single spatial stream inother embodiments. For example, the low bandwidth mode is restricted toan MCS (e.g., MCS0 described above) in which only one spatial stream isutilized. In some of these embodiments, the stream parser 158 is omittedor not utilized. Moreover, the STBC unit 164 and/or CSD units 166 areomitted in some embodiments. Further, in one embodiment where FECencoder 154 is an LDPC encoder rather than a BCC encoder, interleavers160 are omitted. In an embodiment, the same LDPC parity matrix andparameters used for normal mode are also used for low bandwidth mode,and a puncturing/shortening/padding procedure utilizes the values ofN_(CBPS) and N_(DBPS) (number of coded data bits per symbol and uncodeddata bits per symbol, respectively) that correspond to the low bandwidthmode. In some embodiments, padding procedures that are used in the lowbandwidth mode correspond to any such procedures described in U.S.application Ser. No. 13/366,064, filed on Feb. 3, 2012 and entitled“Control Mode PHY for WLAN,” the disclosure of which is herebyincorporated by reference herein in its entirety.

FIGS. 4 and 5 illustrate transmit portions of example PHY processingunits for generating low bandwidth mode data units in embodiments thatuse repetition to decrease the data rate and increase receiversensitivity. For ease of explanation, certain units are not shown inFIGS. 4 and 5 even though the units are in some embodiments included.For example, each of the PHY processing units includes a scrambler, invarious embodiments, such that the information bits input into thetransmit flows depicted in FIGS. 4 and 5 are scrambled bits. In someembodiments, the low bandwidth mode only uses the repetition of FIG. 4or FIG. 5 with BPSK modulation and/or with a single space-time stream,and does not use repetition (e.g., as in the example PHY processing unit150 of FIG. 3) otherwise.

FIG. 4 illustrates an embodiment in which an example PHY processing unit200 generates low bandwidth mode data units utilizing repetition ofBCC-encoded bits, prior to mapping the bits to constellation symbols. ABCC encoder 204 accepts information bits and outputs the BCC-encodedbits to a block encoder 206. The block encoder 206 provides bit-levelrepetition (e.g., [b1 b1, b2 b2, . . . ] for 2× repetition) orblock-level repetition (e.g., [b1 . . . b12, b1 . . . b12, b13 . . .b24, b13 . . . b24, . . . ] for 2× repetition with block size 12), invarious embodiments. In one example embodiment, 2× repetition (rate 1/2block coding) is used. In another example embodiment, 4× repetition(rate 1/4 block coding) is used. The block encoder 206 output couples toa bit flip unit 210 that changes the sign or polarity of select bits(e.g., every other bit) to reduce the peak-to-average power ratio (PAPR)of the generated OFDM signal. In some embodiments, the bit flip unit 210is not included in the PHY processing unit 200.

The output of bit flip unit 210 (or of block encoder 206, if unit 210 isomitted) is coupled to BCC interleaver 212. The BCC interleaver 212 issimilar to interleaver 160 of FIG. 3, in an embodiment. In someembodiments, the BCC interleaver 212 is not included in the PHYprocessing unit 200. The output of BCC interleaver 212 (or of bit flipunit 210 or block encoder 206, if BCC interleaver 212 is omitted) iscoupled to constellation mapper 214. Constellation mapper 214 is similarto constellation mapper 112 of FIG. 2, in an embodiment. Theconstellation size utilized by constellation mapper 214 to generate lowbandwidth mode data units is determined by the MCS mode, which in someembodiments is the lowest MCS (or an MCS lower than the lowest MCS)utilized for normal mode data units, as described above.

The output of the constellation mapper 214 is coupled to an IDFT unit216. The IDFT unit 216 is similar to IDFT unit 172 of FIG. 3 (e.g., usesa 32-point or 16-point IDFT as compared to a 64-point or larger IDFT fornormal mode data units), in an embodiment. The output of IDFT unit 216is coupled to CSD unit 218, in some embodiments. In embodiments orscenarios in which the PHY processing unit 200 operates to generate lowbandwidth mode data units for transmission via multiple transmit chains,the CSD unit 218 inserts a cyclic shift into all but one of the transmitchains to prevent unintentional beamforming. In other embodiments, CSDunit 218 is omitted. The output of CSD unit 218 (or of IDFT unit 216, ifCSD unit 218 is omitted) is coupled to GI insertion and windowing unit220, and the output of GI insertion and windowing unit 220 is coupled toanalog and RF unit 222. In various embodiments and/or scenarios, thegenerated low bandwidth mode data units are then transmitted in a 1 MHzor a 0.5 MHz bandwidth channel (e.g., corresponding to a 32-point or16-point IDFT at unit 216, respectively). In other embodiments, one ormore other suitable channel bandwidths (corresponding to other IDFTsizes) that are less than the minimum normal mode channel bandwidthis/are utilized.

In a more specific example embodiment, where IDFT unit 216 uses a32-point IDFT to generate OFDM symbols having 24 data tones for lowbandwidth mode data units, the BCC encoder 204 is a rate 1/2 BCC encoderthat received 6 information bits per OFDM symbol and outputs 12 bits perOFDM symbol, the block encoder 206 is a rate 1/2 (2× repetition) blockencoder that output 24 bits per OFDM symbol using block-levelrepetition, the 24 output bits are interleaved using a regular BCCinterleaver, and constellation mapping unit 214 utilizes a BPSKmodulation technique.

In one alternative embodiment, block encoder 206 is earlier in thetransmit flow of FIG. 4 than BCC encoder 204 (i.e., repetition of bitsoccurs prior to BCC encoding), and bit flip unit 210 is omitted. Inanother alternative embodiment, block encoder 206 is instead coupled tothe output of constellation mapper 214 (i.e., for repetition ofconstellation points) and bit flip unit 210 is omitted. In some of theselatter embodiments, a phase shift unit (not shown in FIG. 4) is coupledto the block encoder 206 output to reduce the PAPR of the OFDM signal,and the output of the phase shift unit is coupled to the IDFT unit 216.If the phase shift unit is not included in the embodiment, the output ofblock encoder 206 is instead coupled to IDFT unit 216. In variousembodiments, the processing unit 200 is configured to utilize any of therepetition techniques described in U.S. application Ser. No. 13/366,064.

FIG. 5 is a block diagram of a transmit portion of another example PHYprocessing unit 350 for generating low bandwidth mode data units,according to an embodiment. Generally, the various units shown in FIG. 5are similar to the like units in FIG. 4. Unlike the example embodimentof FIG. 4, however, a BCC encoder 352 coupled to the block encoder 354additionally utilizes LDPC encoding, and a stream parser 356. STBC unit360, and spatial mapping unit 362 are included in PHY processing unit350 to support multiple spatial streams and space-time streams.Moreover, in addition to CSD units 364, a second set of CSD units 366 isutilized on each of the space-time streams after the STBC unit 360, inan embodiment. In an embodiment, the second set of CSD units 366 isapplied only if more than one space-time stream is transmitted, in orderto reduce unintended beamforming during the short training field (whichis primarily used to set automatic gain control (AGC) gain at thereceiver). In other embodiments, the CSD units 366 are omitted.Moreover, in some embodiments, the bit flip unit 370 and/or BCCinterleavers 372 is/are omitted. Further, in some embodiments, the blockencoder 354 and bit flip unit 370 are only applied when more than onespace-time stream is being transmitted.

In a more specific example embodiment, where IDFT unit 374 uses a32-point IDFT to generate OFDM symbols having 24 data tones for lowbandwidth mode data units, the BCC/LDPC encoder 352 is a rate 1/2BCC/LDPC encoder that outputs 12×N_(SS) bits per OFDM symbol (whereN_(SS) is the number of spatial streams, the block encoder 354 is a rate1/2 (2× repetition) block encoder that output 24×N_(SS) bits per OFDMsymbol using block-level repetition, and each constellation mapper 376uses BPSK modulation.

In one alternative embodiment, bit repetition occurs after stream parser356 (i.e., in each spatial stream) rather than before stream parser 356.For example, in an embodiment, the block encoder 354 and (if present)bit flip unit 370 are included in each spatial stream, coupled betweenstream parser 356 and the corresponding BCC interleaver 372. As in theembodiment where bit repetition occurs before stream parser 356, the bitrepetition is applied on a bit-by-bit basis in some embodiments, and ona block level in other embodiments.

FIG. 6 is a flow diagram of an example method 400 for generating firstand second data units corresponding to first and second PHY modes,respectively, according to an embodiment. In an embodiment, the firstPHY mode is a normal mode of a long range communication protocol and thesecond PHY mode is a low bandwidth mode of the long range communicationprotocol. For example, in one embodiment, the second PHY mode is acontrol mode. Alternatively, the second PHY mode simply provides rangeextension beyond the first PHY mode. The method 400 is implemented bythe network interface 16 of AP 14 and/or the network interface 27 ofclient station 25-1 of FIG. 1, in various embodiments.

Generally, a first data unit corresponding to the first PHY mode isgenerated at blocks 402 and a second data unit corresponding to thesecond PHY mode is generated at blocks 404. First with reference toblocks 402, a first plurality of information bits is FEC encoded atblock 410. For example, in one embodiment, the first information bitsare BCC encoded. As another example, in an embodiment, the firstinformation bits are LDPC encoded. In some embodiments, at least aportion of the first plurality of information bits corresponds to a dataportion of the first data unit being generated. Further, in someembodiment, an additional portion of the first plurality of informationbits corresponds to a signal field of a preamble of the first data unitbeing generated.

At block 412, the FEC-encoded first information bits are mapped to afirst plurality of constellation symbols. The number of FEC-encoded bitsbeing mapped is greater than the number of information bits prior to FECencoding by a factor related to the coding rate used at block 410. Forexample, if R=1/2 BCC encoding is used at block 410, two FEC-cncodedfirst information bits are produced (and operated on at block 412) foreach information bit operated on at block 410. The constellation mappingat block 412 is similar to the mapping performed by constellationmappers 112 of FIG. 2, in an embodiment. The first plurality ofconstellation symbols corresponds to the particular modulation techniquebeing employed for each OFDM subcarrier. For example, the firstplurality of constellation symbols consists of only +1 and −1 in anembodiment where BPSK modulation is utilized.

At block 414, first OFDM symbols are generated to include the firstconstellation symbols produced at block 412. Each of the first OFDMsymbols utilizes a first tone spacing, and includes a first number ofnon-zero tones that collectively span a first bandwidth. In oneembodiment in which the first data units are long range data units, forexample, the non-zero tones (data and pilot tones) are arrangedaccording to the IEEE 802.11n or IEEE 802.11ac Standard, but with asmaller tone spacing determined by the down-clocking ratio.

Referring next to blocks 404, a second plurality of information bits isFEC encoded at block 416. Block 416 is similar to block 410, in anembodiment. In some embodiments, at least a portion of the secondplurality of information bits corresponds to a data portion of thesecond data unit being generated. Further, in some embodiment, anadditional portion of the second plurality of information bitscorresponds to a signal field of a preamble of the second data unitbeing generated.

At block 420, the FEC-encoded second information are block encoded. Invarious example embodiments, 2× repetition (rate 1/2 block coding) or 4×repetition (rate 1/4 block coding) is used. In one embodiment, the blockencoding at block 420 provides bit-level repetition (e.g., [b1 b1, b2b2, . . . ] for 2× repetition). In another embodiment, the blockencoding at block 420 provides block-level repetition. In this latterembodiment, block 420 includes partitioning the second information bitsinto blocks of n information bits, and repeating each block of ninformation bits m times to generate m*n information bits. For examplethe bit sequence [b1 . . . b12, b1 . . . b12, b13 . . . b24, b13 . . .b24, . . . ] is produced if m=2 (2× repetition) and n=12. In someembodiments, block 420 also includes interleaving the generated m*ninformation bits.

At block 422, the block-encoded second information bits are mapped to asecond plurality of constellation symbols. Block 422 is similar to block412, in an embodiment. The constellation mapping at block 422 is similarto the mapping performed by constellation mapper 214 of FIG. 4 orconstellation mappers 376 of FIG. 5, in various embodiments. The secondplurality of constellation symbols corresponds to the particularmodulation technique being employed for each OFDM subcarrier. Forexample, the second plurality of constellation symbols consists of only+1 and −1 in an embodiment where BPSK modulation is utilized. In someembodiments and/or scenarios, the second plurality of constellationsymbols utilized at block 422 is the same as the first plurality ofconstellation symbols utilized at block 412 (i.e., the modulation typesare the same). In other embodiments and/or scenarios, the s secondplurality of constellation symbols utilized at block 422 is differentthan the first plurality of constellation symbols utilized at block 412(e.g., block 422 uses a modulation type having a smaller set ofconstellation symbols).

At block 424, second OFDM symbols are generated to include the secondconstellation symbols produced at block 422. Each of the second OFDMsymbols utilizes a second tone spacing, and includes a second number ofnon-zero tones that collectively span a second bandwidth. The secondtone spacing is the same as the first tone spacing of the first OFDMsymbols generated at block 414 (i.e. the same clock rate is used atblocks 414 and 424). The second number of non-zero tones is less thanthe first number of non-zero tones in the first OFDM symbols generatedat block 414. The non-zero tones of the second OFDM symbols collectivelyspan a second bandwidth that is less than the first bandwidth of thefirst OFDM symbols.

In some embodiments, the number of non-zero tones in the OFDM symbolsgenerated at block 422 is no more than half the number of non-zero tonesin the OFDM symbols generated at block 414, and the second bandwidth isno more than half the first bandwidth. For example, in one embodimentwhere the second data unit bandwidth is half the first data unitbandwidth, generating the first OFDM symbols includes utilizing a64-point IDFT, and generating the second OFDM symbols includes eitherutilizing a 64-point IDFT and zeroing out at least half of the resultingtones, or utilizing a 32-point IDFT. Example tone maps are shown anddiscussed below in connection with FIGS. 7A and 7B.

In some embodiments, the second data unit is generated using an MCSlower than or equal to the lowest MSC that may be used to generate thefirst data unit. For example, in an embodiment, the FEC encoding atblock 410 is perfor med according to a first MCS selected from aplurality of MCSs corresponding to a plurality of relative throughputlevels, the mapping at block 412 is performed according to the firstMCS, the FEC encoding at block 416 is performed according to a secondMCS corresponding to a relative throughput level lower than or equal toa lowest relative throughput level of the plurality of relativethroughput levels, and the mapping at block 422 is performed accordingto the second MCS.

The method 400 includes additional blocks not shown in FIG. 6, invarious embodiments. In one embodiment, for example, the firstinformation bits are scrambled prior to FEC encoding at block 410 andthe second information bits are scrambled prior to FEC encoding at block416.

FIGS. 7A and 7B are diagrams of example OFDM symbol tone maps 450, 470corresponding to low bandwidth mode data units, according to twoembodiments. More specifically, the tone maps 450, 470 correspond to lowbandwidth mode data units (e.g., in the data and signal field portions)in an embodiment in which normal mode data units correspond todown-clocked IEEE 802.11n or IEEE 802.11ac data units generated using64-point or larger (e.g., 128-, 256-, an/or 512-point) IDFTs. In anembodiment, the tone maps 450, 470 correspond to data units generated bythe PHY processing unit 150 of FIG. 3, the PHY processing unit 200 ofFIG. 4, or the PHY processing unit 350 of FIG. 5, and/or the second PHYmode data units generated in the blocks 404 of method 400 in FIG. 6.

The first example tone map 450 in FIG. 7A corresponds to atone map of alow bandwidth mode OFDM symbol generated using a 32-point IDFT. Of the32 total tones, two sets 452 of non-zero tones correspond to data andpilot tones, a center (zeroed) tone 454 serves as the DC tone, and twosets 458 of tones serve as (zeroed) guard tones. In one exampleembodiment where the 64 tones of normal mode OFDM symbols collectivelyspan a 2 MHz bandwidth, the 32 tones of tone map 450 collectively span a1 MHz bandwidth. Thus, the non-zero tones 452-1 and 452-2 of tone map450 collectively span a bandwidth that is slightly less than half of thebandwidth collectively spanned by the non-zero tones of the 64-pointIDFT normal mode OFDM symbols.

In one embodiment, the non-zero tones 452 include only 24 data tones(e.g., 12 data tones in lower sideband tones 452-1 and 12 data tones inupper sideband tones 452-2) and only two pilot tones (e.g., one pilottone at the +7 index and one pilot tone at the −7 index), the lower-edgeguard tones 458-1 include only three tones, and the upper-edge guardtones 458-2 include only two tones. In some embodiments, the non-zerotones 452 consist of any one of 18, 20, 22, 24, or 26 data tones and anyone of two or four pilot tones, and the guard tones 458 consist of anyone of three or five guard tones. In various different embodiments, moreguard tones 458 are included on the lower edge of tone map 450 than theupper edge, or vice versa. Moreover, the two or four pilot tones are atany set of positions within tone map 450 in various differentembodiments. In some embodiments, the tone map 450 includes more thanone DC tone 454.

The second example tone map 470 in FIG. 7B corresponds to a tone map ofa low bandwidth mode OFDM symbol generated using a 16-point IDFT. Of the16 total tones, two sets 472 of non-zero tones correspond to data andpilot tones, a center (zeroed) tone 474 serves as the DC tone, and twosets 478 of tones serve as (zeroed) guard tones. In one exampleembodiment where the 64 tones of normal mode OFDM symbols collectivelyspan a 2 MHz bandwidth, the 16 tones of tone map 470 collectively span a0.5 MHz bandwidth. Thus, in an embodiment, the non-zero tones 472-1 and472-2 of tone map 470 collectively span a bandwidth that is slightlyless than one-fourth the bandwidth collectively spanned by the non-zerotones of the normal mode OFDM symbols.

In one embodiment, the non-zero tones 472 include only 12 data tones(e.g., six data tones in lower sideband tones 472-1 and six data tonesin upper sideband tones 472-2) and only one pilot tone, the lower-edgeguard tones 478-1 include only one tone, and the upper-edge guard tones478-2 include only one tone. In some embodiments, the non-zero tones 472consist of any one of 11 or 12 data tones and any one of one or twopilot tones, and the guard tones 478 consist of only two guard tones. Invarious different embodiments, the one or two pilot tones are at any setof positions within tone map 470. In some embodiments, the tone map 470includes more than one DC tone 474.

FIG. 8 is a diagram of example normal mode data units 500, 520 havingdifferent bandwidths, according to an embodiment. The normal mode dataunits 500, 520 are down-clocked versions of data units conforming to ashort range protocol. For the particular embodiment shown in FIG. 8, thenormal mode data units 500, 520 are down-clocked versions of IEEE802.11n data units using the “Greenfield” (rather than mixed mode)preamble. In other embodiments, the normal mode data units 500, 520 aredown-clocked versions of data units conforming to other short rangeprotocols. Different examples of normal mode data units according tovarious embodiments are described in U.S. patent application Ser. No.13/359,336.

The normal mode data unit 500 corresponds to a lowest normal modechannel bandwidth (e.g., 2 MHz utilizing a 64-point IDFT), and includesa short training field (STF) 502, a first long training field (LTF1)504, a first signal (SIG1) field 506-1, a second signal (SIG2) field506-2, remaining LTFs 510 (e.g., one additional LTF per spatial stream),and a very high throughput data (VHTDATA) portion 512. Generally, theSTF 502 is used for packet detection, initial synchronization, andautomatic gain control, etc., the LTFs 504 are used for channelestimation and fine synchronization, and the SIG fields 506 are used tocarry certain physical layer (PHY) parameters of the data unit 200, suchas signal bandwidth (e.g., 2 MHz for data unit 500), modulation type,and coding rate used to transmit the data unit, for example.

For higher bandwidth normal mode data units, the STF, LTFs, and SIGfields are duplicated in each of multiple sub-bands, each sub-bandhaving a bandwidth equal to the lowest normal mode channel bandwidth.For example, where data unit 500 is the minimum-bandwidth normal modedata unit and has a 2 MHz bandwidth, data unit 520 duplicates the STF522, LTFs 524, 530, and SIG fields 526 in each 2 MHz band as a preambleto the data portion 532, and the data portion 532 occupies the full (4MHz) bandwidth without frequency duplication. A receiver detectingnormal mode data unit 500 or 520 is able to determine the bandwidth ofthe data unit based on bandwidth information in SIG fields 506 and/orSIG fields 526, in an embodiment.

FIG. 9 is a diagram of a preamble of an example low bandwidth mode dataunit 540, according to an embodiment. The low bandwidth mode data unit540 is generated using the same clock rate as the normal mode data units500, 520, but utilizing a smaller size IDFT to reduce the bandwidth. Forexample, in one embodiment in which the normal mode data units 500 and520 correspond to 2 and 4 MHz bandwidths (generated using 64- and128-point IDFTs), respectively, the low bandwidth mode data unit 540 hasa 1 MHz bandwidth, and is generated using a 32-point IDFT. Similar tothe normal mode data unit 500, the low bandwidth mode data unit 540includes an STF 542, an LTF1 544, a SIG1 field 546-1, a SIG2 field546-2, and remaining LTFs 550 (e.g., one additional LTF per spatialstream, if more than one spatial stream is utilized for low bandwidthmode data units). The low bandwidth mode data unit 540, however, alsoincludes additional SIG fields, in an embodiment. Moreover, in someembodiments, various fields within the preamble of low bandwidth modedata unit 540 differ in various ways from the corresponding fields inthe normal mode data units 500, 520, as described in further detailbelow with reference to FIGS. 10-15. Generally, any of the low rate PHYpreambles described in U.S. application Ser. No. 13/366,064 are utilizedfor low bandwidth mode data units, in various embodiments, but with areduced bandwidth as compared to normal mode data units. In someembodiments, the low bandwidth mode data unit 540 also includes a dataportion (not shown) having the same bandwidth as the preamble of thedata unit 540.

In an alternative embodiment, the SIG fields 506 of the normal mode dataunit 500 (and, in an embodiment, the SIG fields 526 of the widerbandwidth normal mode data unit 520) shown in FIG. 8 is duplicatedacross sub-bands of the normal mode channel, where each sub-band isequal to the bandwidth of the low bandwidth mode data unit. For example,a 2 MHz normal mode data unit includes SIG fields duplicated in two 1MHz sub-bands, in an embodiment, in a manner similar to that shown fornormal mode data unit 520. In an embodiment, other fields (e.g., STF,LTF, and data) are not duplicated across the channel bandwidth. Eachduplicated SIG field has the same format as the SIG field in the lowbandwidth mode data unit, in this embodiment. Moreover, in thisembodiment, additional OFDM symbols are included in the SIG field. Forexample, if in a 64-point IDFT normal mode SIG field would include twoOFDM symbols, a normal mode SIG field with two duplicated 32-point IDFTSIG fields includes four OFDM symbols, in an embodiment. In oneembodiment, a “bandwidth field” is included in a SIG field that is in asub-band commonly used for both 32-point and 64-point IDFT signals.Moreover, in an embodiment, a phase shifter is used on the two sub-bandsto reduce PAPR of the SIF field. In an embodiment, an LTF preceding theduplicated SIG field includes pilot tones in each sub-band that are thesame as the overlapping pilot tones of the low bandwidth mode LTF.

The STF 542 of the low bandwidth mode data unit 540 is used by areceiver for various purposes, including automatic gain control (AGC).The receiver measures the power level of the data unit 540 during theSTF 542, and sets the AGC gain target accordingly to reduce clipping ofthe remainder of the received signal. In one embodiment, however, thepower level of the STF 542 is boosted relative to the rest of the dataunit 540. For example, in one embodiment, the power is boosted by 3 dB.In other embodiments, other suitable levels of power boost are used. Thepower boost facilitates detection of the data unit 540 at the receiver.Moreover, boosting the power of the STF 542 by a suitable amount doesnot tend to cause significant clipping, because the STF 542 generallyincludes fewer non-zero tones, and therefore has a lower PAPR, than theremainder of data unit 540.

In an embodiment, the power boost (e.g., 3 dB power boost) is onlyapplied by a transmitting device for STFs of low bandwidth mode dataunits, and not for STFs of normal mode data units. In anotherembodiment, the power boost (e.g., 3 dB power boost)is only applied by atransmitting device for STFs of low bandwidth mode data units that aremodulated at the lowest data rate (e.g., BPSK modulation, single stream,and with a bit repetition block as shown in FIG. 4 or 5, in anembodiment), and is not applied for STFs of normal mode data unitsand/or for STFs of low bandwidth mode data units that are not modulatedusing bit repetition. While the increased average transmit power levelcan cause a receiver to decrease the AGC gain when receiving a lowbandwidth mode data unit with a power-boosted STF, the increasedquantization error at the analog-to-digital converter (ADC) is generallytolerable due to the robust nature of the low bandwidth mode signal(e.g., a signal having 2× or 4× repetition, using a lowest MCS, etc.) ascompared to normal mode signals.

FIGS. 10 and 11 depict example normal mode and low bandwidth mode STFscorresponding to the normal mode and low bandwidth mode preambles,respectively, of the data units shown in FIGS. 8 and 9, according todifferent embodiments. More specifically, the normal mode STFs of FIGS.10 and 11 correspond to the STFs 502, 522 of FIG. 8 and the lowbandwidth mode STFs of FIGS. 10 and 11 correspond to the STF 542 of FIG.9.

Referring first to FIG. 10, an example normal mode STF 600 includes arepeating first sequence (S) 602, while an example low bandwidth modeSTF 610 includes a repeating second sequence (S1) 612 that is differentthan the first sequence 602. The first sequence 602 and the secondsequence 612 have the same sequence duration/period, but the number ofrepetitions of the first sequence 602 in STF 600 is less than the numberof repetitions of the second sequence 612 in STF 610. For example, in anembodiment, the first sequence 602 is repeated 10 times in STF 600 whilethe second sequence 612 is repeated more than 10 times in STF 610 (i.e.,the low bandwidth mode STF 610 has a longer total duration than thenormal mode STF 600). In another embodiment, the first sequence 602 isrepeated the same number of times as the second sequence 612 (e.g. 10times for each), such that STF 600 and STF 610 have the same totalduration.

The normal mode STF 600 is the same as the STF defined in the IEEE802.11n Standard, in an embodiment. For example, the STF 600 includes 10repetitions of the sequence 602, with five sequences 602 per OFDMsymbol, and with each OFDM symbol having one non-zero tone every fourthtone in the frequency domain (except for a zeroed DC tone), in anembodiment. To achieve the same sequence periodicity as STF 600, STF 610utilizes the same spacing of non-zero tones as STF 600 (e.g., anon-zerovalue every fourth tone, except for the zeroed DC tone), in anembodiment. For example, the OFDM symbols of the low bandwidth mode STF610 (when generated using a 32-point IDFT) include non-zero values foronly tones +/−12, +/−8, and +/−4, in an embodiment. In some embodiments,the non-zero values of these tones are any values that are not equal oralternating (i.e., are not periodic in the frequency domain). Forexample, in an embodiment, the six tone values p(i) are [p(−12), p(−8),p(−4), p(4), p(8), p(12)]=a[sqrt(2), 1+j, sqrt(2)*j, sqrt(2), 1−j,−1−j], where i is the tone index and a is a scaling factor.

In some embodiments, low bandwidth mode data units are generatedutilizing a same IDFT size as the minimum bandwidth normal mode dataunits, but with additional, unused tones being zeroed out. For example,in an embodiment where normal mode data units are generated using atleast a 64-point IDFT, low bandwidth mode data units are also generatedusing a 64-point IDFT, but with one unused sideband of tones beingzeroed out. In these embodiments, the tone map for STF 610 describedabove is shifted to the lower sideband (tones −32 to −1) or to the uppersideband (tones 0 to 31), depending on which of those sub-bands is usedfor low bandwidth mode data units. For example, if the lower sideband isused, the non-zero tones described above as being located at indices+/−12, +/−8, and +/−4 are instead located at indices −28, −24, −20, −12,−8, and −4.

In some embodiments, some additional tones in the OFDM symbols of STF610 are punctured (zeroed out). In various embodiments, between one andfour tones of the six non-zero tones described above are punctured. Forexample, in one embodiment where a 32-point IDFT is used to generate thelow bandwidth mode data units, the +/−12 index tones are punctured sothat four tones remain at +/−8 and +/−4. As another example, in anotherembodiment where a 32-point IDFT is used to generate the low bandwidthmode data units, the +/−8 index tones are punctured so that four tonesremain at +/−12 and +/−4. As another example, in yet another embodimentwhere a 32-point IDFT is used to generate low bandwidth mode data units,the −12 index tone is punctured so that five tones remain at +12, +/−8,and +/−4. By puncturing between one and four of the tones in these orother ways, the first sequences 602 in STF 600 remain periodic withrespect to (e.g., have a period that is an integer multiple of) theperiod of the second sequences 612 in STF 610.

In an alternative embodiment, the sign of every other sequence of thesecond sequences 612 is flipped (e.g., [S1 −S1 S1 −S1 . . . ]), suchthat the effective period of the low bandwidth mode STF 610 is equal totwo times the period of the first sequence 602 of the normal mode STF600 (i.e. a sequence S2 equal to [S1−S1] in STF 610 has twice theduration/period of the sequence 602 in STF 600). In this embodiment, thenormal mode STF 600 has the same STF tone design as defined in the IEEE802.11n and IEEE 802.11ac Standards, with non-zero tones at +/−24,+/−20, +1-16, +/−12, +/−8, and +/−4:

S_(−26,26)=sqrt(½){0,0,1+j,0,0,0,−1−j,0,0,0,1+j,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,0,0,0,0,−1−j,0,0,0,−1−j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0,0,0,1+j,0.0}  Equation12

In this embodiment, the low bandwidth mode STF 610 truncates S_(−26,26)down to the tone values within the indices [−13, 13] in Equation 12above, and shifts the non-zero tones within those indices to the rightor left by two tones (and also, optionally, inserts one new, non-zerovalue in place of the shifted zero DC tone), to achieve the repeatingS1/−S1 pattern. Thus, if the tones are shifted two to the right, thenon-zero tones produced by the 32-point IDFT are at −10, −6, −2, 6, and10 (five non-zero tones), or, if a new tone inserted, at −10, −6, −2, 2,6, and 10 (six non-zero tones). If the tones are instead shifted two tothe left, the non-zero tones produced by the 32-point IDFT are at −10,−6, 2, 6, and 10 (five non-zero tones), or, if a new tone is inserted,at −10, −6, −2, 2, 6, and 10 (six non-zero tones). Any suitable valuemay be used for the new inserted tone, if one is included. Moreover, invarious alternative embodiments, any other suitable non-zero values areused in place of the non-zero values shown in Equation 12.

Referring next to FIG. 11, an example normal mode STF 650 includes arepeating first sequence (S) 652, while an example low bandwidth modeSTF 660 includes a repeating second sequence (S1) 662 that is differentthan the first sequence 652. The second sequence 662 has a sequenceperiod/duration that is half the sequence duration/period of the firstsequence 652, and the number of repetitions of the second sequence 662in STF 660 is greater than twice the number of repetitions of the firstsequence 652 in STF 650. For example, in an embodiment, the firstsequence 652 is repeated 10 times in STF 650 while the second sequence662 is repeated more than 20 times in STF 660 (i.e., the low bandwidthmode STF 660 has a longer total duration than the normal mode STF 650).In another embodiment, the second sequence 662 is repeated exactly twicethe number of times as the first sequence 652 (e.g., 20 and 10 times,respectively), such that STF 660 and STF 650 have the same totalduration.

The normal mode STF 650 is the same as the STF defined in the IEEE802.11n Standard, in an embodiment. For example, the STF 650 includes 10repetitions of the sequence 652, with five sequences 652 per OFDMsymbol, and with each OFDM symbol having one non-zero tone every fourthtone in the frequency domain (except for a zeroed DC tone), in anembodiment. To achieve half the sequence periodicity of STF 650, STF 660utilizes twice the spacing of non-zero tones as STF 600 (e.g., anon-zerovalue every eighth tone, except for the zeroed DC tone), in anembodiment. For example, the OFDM symbols of the low bandwidth mode STF660 (when generated using a 32-point IDFT) include non-zero values foronly two tones (+/−8), in an embodiment. In some embodiments, thenon-zero values of these tones are any values that satisfy the criteriap(−8)!=p(8) and p(−8)!=−p(8). For example, in an embodiment, the twotone values p(i) are [p(−8), p(8)]=a[sqrt(2), 1+j], where i is the toneindex and a is a scaling factor.

In an embodiment where normal mode data units are generated using atleast a 64-point IDFT, and where low bandwidth mode data units are alsogenerated using a 64-point IDFT but with one unused sideband of tonesbeing zeroed out, the tone map for STF 660 described above is shifted tothe lower sideband (tones −32 to −1) or to the upper sideband (tones 0to 31), depending on which of those sub-bands is used for low bandwidthmode data units. For example, if the lower sideband is used, thenon-zero tones described above as being located at indices +/−8 areinstead located at indices −24 and −8. If the upper sideband is used,the non-zero tones are instead located at indices +8 and +24.

In an alternative embodiment, the sign of every other sequence of thesecond sequences 662 is flipped (e.g., [S1 −S1 S1 −S1 . . . ]), suchthat the effective period of the low bandwidth mode STF 660 is equal tothe period of the first sequence 652 of the normal mode STF 650 (i.e., asequence S2 equal to [S1−S1] in STF 660 has the same duration/period asthe sequence 662 in STF 660). In this embodiment, only tones +/−12 and+/−4 have non-zero values. It is noted that this embodiment correspondsto one of the puncturing embodiments described above with reference toFIG. 10, in a specific case in which the tones +/−8 are punctured. Insome embodiments, the non-zero values of the tones are any values thatare not equal or alternating (i.e., are not periodic in the frequencydomain). For example, in an embodiment, the four tone values p(i) are[p(−12), p(−4), p(4), p(12)]=a[−j, −1−j, 1+j, −1−j], where i is the toneindex and a is a scaling factor. This tone map is shifted down or up 16tones in embodiments where low bandwidth mode data units are generatedusing a 64-point IDFT but with one unused sideband of tones zeroed out.For example, if the lower sideband is used, the non-zero tones describedabove as being located at indices +/−12 and +/−4 are instead located atindices −28, −20, −12, and −4.

Different designs may be utilized for the tone map of the normal modeSTF (e.g., STF 602 of FIG. 10 or STF 652 of FIG. 11), as described belowin various sets of embodiments. It is understood that some of the setsof embodiments described below may overlap in scope with certain othersets of embodiments described below, and/or may overlap in scope withcertain normal mode STFs shown in FIGS. 10 and 11.

In one set of embodiments, and regardless of which low bandwidth modeSTF described above is utilized, the normal mode STF is unchanged fromthe IEEE 802.11a/n STF, i.e., non-zero values are only at tones +/−24,+/−20, +/−16, +/−12, +/−8, and +/−4 of the 64-point IDFT. Any suitablevalues are used for the non-zero tones such that the non-zero values ofthe tones are not equal or alternating (i.e., are not periodic in thefrequency domain). Alternatively, in an embodiment, the tones at indices+/−28 also have non-zero values. In either of these embodiments, anysuitable values are used for the non-zero tones such that the non-zerovalues of the tones are not equal or alternating (i.e. are not periodicin the frequency domain).

In another set of embodiments, where the low bandwidth mode STF tonesare arranged according to either of the embodiments described above withreference to FIG. 11 (e.g., non-zero tones at +/−8 or non-zero tones at+/4 and +/−12), the normal mode STF tone plan is complementary to thelow bandwidth mode STF tone plan. In these embodiments, the normal modeSTF includes non-zero tones at +/−24, +/−20, +/−16, +/−12, +/−8, and+/−4 of the 64-point IDFT, except that any tones that would align with alow bandwidth mode STF tone (if duplicated in each sideband of the64-point IDFT tone set) are zeroed out. For example, in the aboveembodiment where the low bandwidth mode includes only non-zero values at+/−8 of the 32-point IDFT tones (which translates to indices +/−24 and+/−12 of the 64-point IDFT tones), the normal mode STF includes non-zerovalues only at indices +/−20, +/−16, +/−8, and +/−4. As another example,in the above embodiment where the low bandwidth mode includes onlynon-zero values at +/−12 and +/−4 of the 32-point IDFT tones (whichtranslates to indices +/−28, +/−20, +/−12, and +/−4 of the 64-point IDFTtones), the normal mode STF includes non-zero values only at indices+/−24, +/−16, and +/−8. These complementary designs provide separationbetween the normal mode and low bandwidth mode STFs. Thus, a receiver ismore easily able to auto-detect the PHY mode based on the STF by runningcross-correlators of one period of the normal mode STF sequence and oneperiod of the low bandwidth mode STF sequence, with minimalcross-trigger due to the non-overlapping tones.

In yet another set of embodiments, the normal mode STF tone map isdesigned in conjunction with the low bandwidth mode STF in any suitablemanner such that the normal mode and low bandwidth mode STFs have boththe same sequence periodicity and the same STF duration (i.e. the samenumber of OFDM symbols in the STF). Any of the normal mode STFembodiments described above and any of the low bandwidth mode STFembodiments described above that satisfy these criteria are included inthis set of embodiments. These embodiments include normal mode STFshaving non-zero tones at indices that are the same as, partiallyoverlap, or are complementary to the tones of the low bandwidth mode STF(if the low bandwidth mode STF were duplicated in the lower and uppersidebands of the normal mode tone set). For example, in an embodiment,the normal mode (64-point IDFT) STF includes non-zero tones only atindices +/−24, +/−20, +/−16, +/−12, +/−8, and +/−4, while the lowbandwidth mode (32-point IDFT) STF includes non-zero tones only atindices +/−12, +/−8, and +/−4. In this example embodiment, the tonesonly partially overlap but result in STF sequences having the sameperiodicity. As another example, in an embodiment, the tones arecomplementary (as described above), and the normal mode STF duration isgreater than the low bandwidth mode STF duration.

In some of these embodiments, because both the normal mode and lowbandwidth mode STFs have the same periodicity and duration in the timedomain, a receiving device does not auto-detect the PHY mode during theSTF, and instead performs unified STF processing. The receiver insteadperforms auto-detection of the PHY mode during the LTF and/or SIG field,as discussed below, in these embodiments.

Alternatively, in an embodiment, the normal mode STF and low bandwidthmode STF have different durations. For example, in an embodiment, thenormal mode (64-point IDFT) STF includes non-zero tones only at indices+/−24, +/−20, +/−16, +/−12, +/−8, and +/−4, the low bandwidth mode(32-point IDFT) STF includes non-zero tones only at indices +/−12, +/−8,and +/−4 (i.e., the normal mode and low bandwidth mode STFs have thesame periodicity), and the low bandwidth mode STF has a longer durationthan the normal mode STF (e.g., as shown in FIG. 10).

With reference again to FIGS. 10 and 11, a receiving device, in anembodiment, includes cross-correlators corresponding to the firstsequence 602 or 652 and the second sequence 612 or 662 to automaticallydetect the PHY mode of a received data unit, i.e., to detect whcther thereceived data unit is a normal mode or a low bandwidth mode data unit.In another embodiment, where normal mode and low bandwidth mode STFshave different (larger or smaller) periodicities of repeating sequences,a receiving device auto-detects the PHY mode of a received data unit bydetermining which periodicity is used. In yet another embodiment, areceiving device auto-detects the PHY mode of a received data unit bydetecting the presence or absence of energy in one or more sub-bandswithin a channel. For example, in one embodiment where the normal modeSTF 600 occupies a 2 MHz channel, and where the low bandwidth mode STF610 occupies a 1 MHz sub-band at a location within the 2 MHz channelthat is known a priori by the receiving device, the receiver determinesthat a received data unit is a normal mode data unit if the STF signalenergy is detected across the entire 2 MHz channel, and is a lowbandwidth mode data unit if the STF signal energy is detected only inthe known 1 MHz sub-band.

In still other embodiments, the STFs of received data units are not usedto auto-detect the PHY mode. FIGS. 12-17 illustrate preamble designs andmethod flow diagrams utilized when auto-detection of the PHY mode is notbased on the STF, but instead based on a second preamble portion (e.g.,based on an LTF and/or SIG field). In other embodiments, auto-detectionof PHY mode based on the second preamble portion according to any one ofthe designs and/or methods described below is used in conjunction withauto-detection of PHY mode based on the STF according to any one of thedesigns and/or methods described above with reference to FIGS. 10 and11.

FIG. 12 is a diagram of an example second preamble portion 700 of anormal mode data unit and an example second preamble portion 720 of alow bandwidth mode data unit, according to an embodiment. In oneembodiment, the second preamble portion 700 corresponds to LTF1 504 andSIG1 506-1 of the normal mode data unit 500 in FIG. 8, and the secondpreamble portion 720 corresponds to at least a portion of LTF1 544 ofthe low bandwidth mode data unit 540 in FIG. 9. In various embodiments,the second preamble portion 700 is included in the same preamble as anyone of the normal mode STFs discussed above with reference to FIGS. 10and 11, and the second preamble portion 720 is included in the samepreamble as any corresponding one of the low bandwidth mode STFsdiscussed above with reference to FIGS. 10 and 11. For example, in oneembodiment, the second preamble portion 700 follows the STF 600 of FIG.10 and the second preamble portion 720 follows the STF 610 of FIG. 10.

The second preamble portion 700 includes a double guard interval (DGI)702, two long training symbols (LTS) 704 in a first long training field(LTF1), a guard interval (GI) 706, and a first signal field (SIG1) 708.The first OFDM symbol of the SIG1 field 708 begins a time interval 730after the beginning of LTF1 (i.e., the beginning of DGI 702 withinLTF1). The second preamble portion 720 similarly includes DGI 722, twoLTS 724 in LTF1, and a guard interval (GI) 726. The LTF1 of the secondpreamble portion 720, however, includes a greater number of longtraining symbols than the second preamble portion 700 of the normal modedata unit. For example, LTF1 of the second preamble portion 720 includesfour long training symbols, in an embodiment. In one embodiment, eachlong training symbol after LTS 724-2 is preceded by a guard interval.For example, as seen in the example embodiment of FIG. 12, the guardinterval 726 separates the third and fourth LTSs 724-2 and 724-3,respectively. By including guard interval 726, the location of the thirdLTS 724-3 relative to the beginning of LTF1 of preamble portion 720 isthe same as the location of the SIG1 field 708 relative to the beginningof LTF1 of preamble portion 700 (i.e., each begins a time interval 730after the beginning of the corresponding LTF1). Moreover, the SIG1 field708 is modulated with a different modulation technique than the thirdLTS 724-3, in an embodiment. For example, the SIG1 field 708 isquaternary binary phase shift key (QBPSK) modulated and the third LTS724-3 is binary phase shift key (BPSK) modulated, or vice versa, invarious embodiments. Thus, a receiving device that synchronizes with areceived data unit prior to the SIG1 field 708 or third LTS 724-3 candetect the modulation technique being used at the location of SIG1 (if anormal mode data unit) or the third LTS (if a low bandwidth mode dataunit), and determine the PHY mode accordingly. FIG. 13 illustrates theBPSK modulation constellation 750 and the QBPSK modulation constellation760. As seen in FIG. 13, the set of two constellation symbols for QBPSKis rotated by 90 degrees with respect to the set of two constellationsymbols for BPSK.

FIG. 14 is a flow diagram of an example method 800 for generating afirst preamble for a first data unit corresponding to a first PHY modeand a second preamble for a second data unit corresponding to a secondPHY mode different than the first PHY mode, according to an embodiment.In an embodiment, the first PHY mode is a normal mode of a long rangecommunication protocol and the second PHY mode is a low bandwidth modeof the long range communication protocol. For example, in oneembodiment, the second PHY mode is a control mode. Alternatively, thesecond PHY mode simply provides range extension beyond the first PHYmode. The method 800 is implemented by the network interface 16 of AP 14and/or the network interface 27 of client station 25-1 of FIG. 1, invarious embodiments.

Generally, a first data unit corresponding to the first PHY mode isgenerated at blocks 802 of FIG. 14, and a second data unit correspondingto the second PHY mode is generated at blocks 804 of FIG. 14. First withreference to blocks 802, an STF of the first data unit is generated atblock 810. An LTF that follows the STF is generated at block 812, and aSIG field that follows the LTF is generated at block 814. In someembodiments, the preamble generated at blocks 802 also includesadditional fields (e.g., additional LTFs, SIG fields, etc.). The STFgenerated at block 810 includes a repeating first sequence. The STF issimilar to the STF 600 of FIG. 10, or the STF 650 of FIG. 11, in variousembodiments. In an embodiment, the LTF generated at block 812 includesmultiple OFDM symbols (e.g., two OFDM symbols). The SIG field generatedat block 814 provides information for interpreting the first data unit,such as signal bandwidth, modulation type, and/or coding rate, forexample. Moreover, the SIG field includes a first OFDM symbol that ismodulated according to a first modulation technique to indicate to areceiver that the first data unit corresponds to the first PHY mode. Thefirst OFDM symbol begins a time interval T₁, and ends a time intervalT₂, after the LTF generated at block 812 begins.

Referring next to blocks 804, an STF of the second data unit isgenerated at block 820, and an LTF that follows the STF is generated atblock 822. In some embodiments, the preamble generated at blocks 804also includes additional fields (e.g., additional LTFs, SIG fields,etc.). The STF has a duration greater than the duration of the STFgenerated at block 810, and includes a repeating second sequencedifferent than the repeating first sequence in the STF generated atblock 810. Moreover, the period of the repeating second sequence isequal to the period of the repeating first sequence. The LTF generatedat block 822 includes a second OFDM symbol modulated according to asecond modulation technique different than the first modulationtechnique used to modulate the first OFDM symbol of the SIG fieldgenerated at block 814. The second modulation technique indicates to areceiver that the second data unit corresponds to the second PHY mode.In an embodiment, the first modulation technique of the method 800 isone of BPSK modulation and QBPSK modulation, and the second modulationtechnique of the method 800 is the other of BPSK modulation and QBPSKmodulation. The second data unit LTF generated at block 822 includesmore long training symbols (OFDM symbols) than the first data unit LTFgenerated at block 812. In one embodiment where the first data unit LTFincludes two OFDM symbols, for example, the second data unit LTFincludes four OFDM symbols. The second OFDM symbol at least partiallyoccupies a location in the second preamble that begins the time intervalT₁ after, and ends the time interval T₂ after, the LTF generated atblock 822 begins. In one embodiment, the second OFDM symbol begins thetime interval T₁ after, and ends the time interval T₂ after, the LTFgenerated at block 822 begins. In some embodiments, the second OFDMsymbol is the long training symbol 724-3 in preamble portion 720 of FIG.12 (i.e., a long training symbol that follows two earlier long trainingsymbols in the LTF). By coordinating the timing of the first OFDM symbolwithin the first data unit preamble (e.g., relative to the start of theLTF of the first data unit preamble) and similarly coordinating thetiming of the second OFDM symbol within the second data unit preamble(e.g., relative to the start of the LTF of the second data unitpreamble), a receiving device with a priori knowledge of the timing candetect the modulation type used during the relevant time period todetermine the PHY mode of a received data unit.

In some embodiments, the first OFDM symbol of the SIG field generated atblock 814 is immediately preceded by a guard interval, e.g., as in thepreamble portion 700 illustrated in FIG. 12, and the second OFDM symbolof the LTF field generated at block 822 is immediately preceded byanother guard interval, e.g., as in the preamble portion 720 illustratedin FIG. 12. In one such embodiment, the two guard intervals have thesame duration.

In some embodiments, the method 800 further includes (within the blocks802 for generating the first preamble) generating a second SIG fieldfollowing the SIG field generated at block 814. The second SIG fieldincludes a third OFDM symbol modulated according to either a thirdmodulation technique to indicate to a receiver that the first data unitis a single-user data unit, or a fourth modulation technique differentthan the third modulation technique to indicate to a receiver that thefirst data unit is a multi-user data unit. In one embodiment, the thirdmodulation technique is one of BPSK and QBPSK, and the fourth modulationtechnique is the other of BPSK and QBPSK.

FIG. 15 is a diagram of another example second preamble portion 850 of anormal mode data unit and another example second preamble portion 870 ofa low bandwidth mode data unit, according to an embodiment. The secondpreamble portions 850, 870 are the same as the second preamble portions700, 730 of FIG. 12, in an embodiment, except that the third longtraining OFDM symbol 724-3 of low bandwidth mode preamble portion 720 isreplaced by a SIG field 878. Thus, the PHY mode is indicated by themodulation technique (e.g., QBPSK or BPSK) in the first SIG field,regardless of whether the received data unit is a normal mode or lowbandwidth mode data unit.

FIG. 16 is a diagram of an example second preamble portion 900 of anormal mode, single-user data unit and an example second preambleportion 920 of a normal mode, multi-user data unit, according to anembodiment. In one embodiment, the second preamble portions 900, 920each correspond to LTF1 504, SIG1 506-1, and SIG2 506-2 of the normalmode data unit 500 in FIG. 8. In various embodiments, the secondpreamble portions 900, 920 are each included in the same preamble as anyone of the normal mode STFs discussed above with reference to FIGS. 10and 11. For example, in one embodiment, the second preamble portion 900or 920 (depending on whether the data unit is single- or multi-user)follows the STF 600 of FIG. 10.

The second preamble portion 900 of the normal mode, single-user dataunit includes a double guard interval 902, long training symbols 904,guard interval 906, and first SIG field 908-1 similar to the doubleguard interval 702, long training symbols 704, guard interval 706, andfirst SIG field 708, respectively, of the second preamble portion 700 inFIG. 12, in an embodiment. As with the second preamble portion 700 inFIG. 12, the modulation type of the first SIG field 908-1 indicates to areceiver the PHY mode of the data unit (i.e., in the embodiment shown,QBPSK modulation is used to indicate a normal mode data unit). Thesecond preamble portion 900 additionally includes a second guardinterval 910 following the first SIG field 908-1, and a second SIG field908-2 following the guard interval 910. The second SIG field 908-2 ismodulated using a modulation technique that indicates whether the dataunit with preamble portion 900 is a single-user or multi-user data unit.In the example embodiment shown in FIG. 16, the second SIG field 908-2is QBPSK-modulated to indicate that the preamble portion 900 is asingle-user data unit.

The second preamble portion 920 of the normal mode, multi-user data unitsimilarly includes a double guard interval 922, long training symbols924, guard interval 926, and first SIG field 928-1. Again, themodulation type of the first SIG field 928-1 is used to indicate the PHYmode of the data unit (i.e., in the embodiment shown, QBPSK modulationis used to indicate a normal mode data unit), and the modulation type ofthe second SIG field 928-2 is used to indicate whether the data unitwith preamble portion 920 is a single-user or multi-user data unit. Inthe example embodiment shown in FIG. 16, the second SIG field 928-2 isBPSK-modulated to indicate that the preamble portion 920 is a multi-userdata unit. In other embodiments, BPSK modulation in the second SIG fieldindicates a single-user data unit and QBPSK modulation in the second SIGfield indicates a multi-user data unit. In still other embodiments, anyother suitable modulation techniques are used in the second SIG field todistinguish single-user and multi-user data units.

In an alternative embodiment, the modulation type of a SIG field afterthe second SIG field is used to indicate whether a data unit issingle-user, or multi-user. While only normal mode data units are shownin FIG. 16, both normal mode and low bandwidth mode data units can beeither single-user or multi-user data units, in some embodiments. Forexample, in one embodiment, the modulation type of a first SIG field ofa low bandwidth mode data unit is used to indicate whether a lowbandwidth data unit is single-user, or multi-user. As another example,in an embodiment where the modulation type of the first SIG field of alow bandwidth mode data unit is used to indicate PHY mode, themodulation type of a second SIG field is used to indicate whether thedata unit is single-user or multi-user. In other embodiments, lowbandwidth mode data units are not permitted to be multi-user data units.

In another embodiment, whether a data unit is single-user or multi-useris indicated to a receiver by a special “SU/MU bit” in a SIG ficld ofnormal mode data units (and/or of low bandwidth mode data units, ifpermitted to be multi-user). The SU/MU bit is in the first SIG field(e.g., of two SIG fields) of the data unit, in an embodiment. In some ofthese embodiments, multi-user data units include an extended lengthmulti-user STF (MUSTF) after the second SIG field, which provides areceiver with more time to adjust automatic gain control after decodingthe SIG field of a multi-user data unit. In some embodiments, multi-userdata units are so-called “long preamble data units” that use a preamblestructure similar to the IEEE 802.11n mixed-mode preamble, andsingle-user data units are so-called “short preamble data units” thatuse a (shorter) preamble similar to the IEEE 802.11n Greenfieldpreamble.

FIG. 17 is a flow diagram of an example method 1000 for generating afirst preamble for a first data unit corresponding to a first PHY modeand a second preamble for a second data unit corresponding to a secondPHY mode different than the first PHY mode, according to an embodiment.In an embodiment, the first PHY mode is a normal mode of a long rangecommunication protocol and the second PHY mode is a low bandwidth modeof the long range communication protocol. For example, in oneembodiment, the second PHY mode is a control mode. Alternatively, thesecond PHY mode simply provides range extension beyond the first PHYmode. The method 1000 is implemented by the network interface 16 of AP14 and/or the network interface 27 of client station 25-1 of FIG. 1, invarious embodiments.

Generally, a first data unit corresponding to the first PHY mode isgenerated at method portion 1002 and a second data unit corresponding tothe second PRY mode is generated at method portion 1004. First withreference to method portion 1002, an LTF of the first data unit isgenerated at block 1010. A first SIG field that follows the LTF isgenerated at block 1012, and a second SIG field that follows the firstSIG field is generated at block 1014. In some embodiments, the preamblegenerated at blocks 1002 also includes additional fields (e.g., an STF,additional LTFs, additional SIG fields, etc.). In an embodiment, the LTFgenerated at block 1010 includes multiple OFDM symbols (e.g., two OFDMsymbols). The first and second SIG fields generated at blocks 1012 and1014, respectively, each provide information for interpreting the firstdata unit, such as signal bandwidth, modulation type, and/or codingrate, for example. The first SIG field includes a first OFDM symbol thatis modulated according to a first modulation technique to indicate to areceiver that the first data unit corresponds to the first PHY mode. Thefirst OFDM symbol begins a time interval T₁, and ends a time intervalT₂, after the LTF generated at block 1010 begins. The second SIG fieldincludes a second OFDM symbol that is modulated according to either asecond modulation technique to indicate to a receiver that the firstdata unit is a single-user data unit, or a third modulation techniquedifferent than the second modulation technique to indicate to a receiverthat the first data unit is a multi-user data unit.

Referring next to method portion 1004, an LTF of the second data unit isgenerated at block 1020. In some embodiments, the preamble generated atmethod portion 1004 also includes additional fields (e.g., an STF,additional LTFs. SIG fields, etc.). The LTF includes a third OFDM symbolmodulated according to a fourth modulation technique different than thefirst modulation technique used to modulate the first OFDM symbol of theSIG field generated at block 1014. The fourth modulation techniqueindicates to a receiver that the second data unit corresponds to thesecond PHY mode. The third OFDM symbol at least partially occupies alocation in the second preamble that begins the time interval T₁ after,and ends the time interval T₂ after, the LTF generated at block 1020begins. In one embodiment, the third OFDM symbol begins the timeinterval T₁ after, and ends the time interval T₂ after, the LTFgenerated at block 1020 begins. Similar to the method 800 of FIG. 14,coordinating the timing of the first OFDM symbol within the first dataunit preamble (e.g., relative to the start of the LTF of the first dataunit preamble) and similarly coordinating the timing of the third OFDMsymbol within the second data unit preamble (e.g., relative to the startof the LTF of the second data unit preamble) allows a receiving devicewith a priori knowledge of the timing to detect the modulation type anddetermine the PHY mode of a received data unit.

In an embodiment, the first modulation technique of the method 1000 isone of BPSK modulation and QBPSK modulation, and the fourth modulationtechnique of the method 1000 is the other of BPSK modulation and QBPSKmodulation. Moreover, in an embodiment, the second modulation techniqueof the method 1000 is one of BPSK modulation and QBPSK modulation, andthe third modulation technique of the method 1000 is the other of BPSKmodulation and QBPSK modulation.

The long training symbols (LTSs) of the second preamble portionsdescribed above with reference to FIGS. 12-17 are defined in variousways according to different embodiments. In one embodiment, each LTS inLTF1 544 is as defined in the IEEE 802.11n Standard, i.e., with +1 or −1in an arbitrary order. Moreover, in an embodiment, the tones of thenormal mode (e.g., 64-point IDFT) LTS have the same values as thecorresponding tones that would result if the low bandwidth mode (e.g.,32-point IDFT) LTS were replicated in each of the lower and uppersidebands of the 64-point IDFT. In this embodiment, the remaining tonesnot occupied by the replicated 32-point IDFT tones (e.g., four extratones, if the normal mode LTS has 56 data/pilot tones and the lowbandwidth mode LTS has 26 data/pilot tones) are filled in with othersuitable values. This design provides convenience of frequency domainauto-detection (which is always performed in the 32-point IDFThalf-band, even if the signal is a 64-point or greater IDFT signal, inan embodiment). In some embodiments, to reduce PAPR, the normal modesignal includes a phase shift (e.g., 90 degrees) across all tones in theupper sideband of the LTS or across all tones in the lower sideband ofthe LTS. Moreover, if any tone re-routing is utilized as a result of lowbandwidth mode signals being duplicated in the frequency domain acrossthe normal mode channel bandwidth (as described below with reference toFIGS. 22, and 23), the corresponding LTS tones are adjusted in a likemanner.

In some embodiments, communication channels of a WLAN (e.g., WLAN 10 ofFIG. 1) are defined based on normal mode signal bandwidths only, whereaslow bandwidth mode signals (e.g., control mode signals, in anembodiment) are transmitted in one or more frequency bands within thosecommunication channels. For example, the channelization on which mediumaccess control (MAC) protocols operate corresponds to the set ofchannels used to transmit normal mode signals, in an embodiment. In amore specific example embodiment, where normal mode signals aretransmitted in 2 MHz, 4 MHz, 8 MHz, or 16 MHz bandwidths (e.g.,corresponding to data units generated using 64-point, 128-point,256-point, or 512-poin IDFTs), the defined channels are 2 MHz, 4 MHz, 8MHz, or 16 MHz channels, and a low bandwidth mode signal having a 1 MHzbandwidth (e.g., corresponding to a data unit generated using a 32-pointIDFT) is transmitted in a 1 MHz band within one of the 2 MHz channels.In the discussions below referring to FIGS. 18-23, for ease ofexplanation and unless otherwise indicated, normal mode data units willbe assumed to be data units generated using a 64-point IDFT as a minimumIDFT size, corresponding to a minimum 2 MHz channel bandwidth. In otherembodiments, however, the minimum IDFT size and/or bandwidth may beanother suitable value, with various other system parameters (e.g., thelow bandwidth mode bandwidth and IDFT size) being scaled or otherwisemodified accordingly.

Various placements of the frequency band for low bandwidth mode signalswithin normal mode channels are described below with reference to FIGS.18-21. In each of FIGS. 18-21, the channels 1100 are used to transmitnormal mode data units. Each channel 1100 has a bandwidth equal to the 2MHz minimum bandwidth of normal mode signals, in an embodiment. Whilethree channels 1100 are shown in each of FIGS. 18-21, one, two, four, orgreater than four channels 1100 are utilized in other embodiments.Moreover, in some embodiments, two or more of the channels 1100 can becombined to form a composite channel (e.g., 4 MHz, 8 MHz, etc.), subjectto any combination criteria or rules. While frequency band placement isshown with respect to the second channel 1100-2 in FIGS. 18-21, otherscenarios may involve placement within channel 1100-1, 1100-3, or anyother suitable channel 1100.

FIG. 18 is a diagram of an example placement of a frequency band used totransmit a low bandwidth mode signal 1104 within the communicationchannel 1100-2, according to an embodiment. In one embodiment, the lowbandwidth mode signal 1104 is a 1 MHz wide, 32-point IDFT signal (or a64-point IDFT signal with the appropriate tones zeroed out) thatincludes OFDM symbols having the tone map 450 of FIG. 7A (e.g., in thedata, SIG, and/or LTF portions of the data units). In anotherembodiment, the low bandwidth mode signal 1104 is a 0.5 MHz wide,16-point IDFT signal (or a 64-point IDFT signal with the appropriatetones zeroed out) that includes OFDM symbols having the tone map 470 ofFIG. 7B. In still other embodiments, the low bandwidth mode signal 1104is generated using another suitable IDFT size and occupies anothersuitable bandwidth less than 2 MHz.

In the embodiment and scenario shown in FIG. 18, the low bandwidth modesignal 1104 is transmitted in a frequency band that is fixed at thecenter of channel 1100-2. More generally, in an embodiment, MAC layeroperations (e.g., implemented by MAC processing unit 18 and/or MACprocessing unit 28 of FIG. 1, in various embodiments) require that thelow bandwidth mode signal 1104 be transmitted in a frequency bandcentered within any one of communication channels 1100. By centering theband within one of channels 1100, interference with other channels 1100is generally reduced.

FIG. 19 is a diagram of another example placement of a frequency bandused to transmit a low bandwidth mode signal 1106 within thecommunication channel 1100-2, according to an embodiment. The signal1106-1 is transmitted in a frequency band corresponding to the lowersideband of channel 1100-2, and a duplicate of signal 1106-1 (i.e.,signal 1106-2) is simultaneously transmitted in a frequency bandcorresponding to the upper sideband of channel 1100-2. In an embodiment,each low bandwidth mode signal 1106 is a 1 MHz wide signal that includesOFDM symbols having the tone map 450 of FIG. 7A (e.g., in the data, SIG,and/or LTF portions of the data units). In various embodiments, thecombination of signals 1106-1 and 1106-2 is generated using two 32-pointIDFTs, or using one 64-point IDFT. In an embodiment where the signalincludes OFDM symbols having the tone map 450 of FIG. 7A, three guardtones are included at the lower edge of channel 1100-2, while only twoguard tones are included at the upper edge of channel 1100-2. In anotherembodiment, each low bandwidth mode signal 1106 is instead a 0.5 MHzwide signal that includes OFDM symbols having the tone map 470 of FIG.7B, and four copies of the signal 1106 are transmitted within channel1100-2. In various embodiments, the combination of four signals 1106 isgenerated using four 16-point IDFTs, or using one 64-point IDFT. In yetanother embodiment, each low bandwidth mode signal 1106 is 0.5 MHz widesignal that includes OFDM symbols having the tone map 470 of FIG. 7B,and only two copies of the signal 1106 are transmitted within channel1100-2. In one such embodiment, the two copies of the 0.5 MHz signal arelocated within a centered 1 MHz band within the channel 1100-2, tominimize interference with other channels.

Generally, embodiments that include duplication of the low bandwidthmode signal 1106 in multiple frequency bands provide frequencydiversity. For example, a receiving device can perform frequency domaincombining/averaging of the duplicated signals 1106. Moreover, phaseshifts are applied across duplicates of the low bandwidth mode signal1106, in some embodiment (e.g., [1 −1 −1 −1] for 4× frequencyduplication, or [1 j] for 2× frequency duplication) to reduce PAPR.

FIG. 20 is a diagram of another example placement of a frequency bandused to transmit a low bandwidth mode signal 1110 within thecommunication channel 1100-2, according to an embodiment. In oneembodiment, the low bandwidth mode signal 1110 is a 1 MHz wide, 32-pointIDFT signal (or a 64-point IDFT signal with the appropriate tones zeroedout) that includes OFDM symbols having the tone map 450 of FIG. 7A(e.g., in the data, SIG, and/or LTF portions of the data units). In theembodiment and scenario shown in FIG. 20, the low bandwidth mode signal1110 is transmitted in a frequency band that is fixed in the lowersideband of channel 1100-2. More generally, in an embodiment in whichthe low bandwidth mode signal 1110 includes OFDM symbols having morelower-edge guard tones than upper-edge guard tones (e.g., the tone map450 of FIG. 7A), MAC layer operations (e.g., implemented by MACprocessing unit 18 and/or MAC processing unit 28 of FIG. 1, in variousembodiments) do not permit the low bandwidth mode signal 1110 to betransmitted in an upper sideband of any of communication channels 1100.In this manner, interference with other channels may generally bereduced, and the filter design requirement may also be relaxed. In otherembodiments, where the low bandwidth mode signal 1110 tone map insteadhas more upper-edge guard tones than lower-edge guard tones, the MAClayer operations do not permit the low bandwidth mode signal 1110 to betransmitted within the lower sideband of any of communication channels1100.

In some embodiments where the low bandwidth mode frequency band isrestricted to a particular (lower or upper) sideband of a not mal modechannel, a receiver auto-detects the PHY mode based on the signal (orsignal portion) detected in the frequency band, where the frequency bandlocation is known a priori to the receiver. For example, in anembodiment, the receiver knows that a low bandwidth mode (e.g., controlmode) signal will only be transmitted in a lower sideband of a normalmode channel. Accordingly, for purposes of auto-detecting the PHY mode(e.g., based on STF differences, etc.), the receiver only observessignals in the lower sideband of the channel, in this embodiment.Conversely, the receiver detects the bandwidth of different normal modedata units (e.g., 2 MHz, 4 MHz, 8 MHz, etc.) based on a signal field(e.g., an HTSIG field as used in IEEE 802.11n and IEEE 802.11ac), in anembodiment.

FIG. 21 is a diagram of yet another example placement of a frequencyband used to transmit a low bandwidth mode signal 1112 within thecommunication channel 1100-2, according to an embodiment. In oneembodiment, the low bandwidth mode signal 1112 is a 0.5 MHz wide,16-point IDFT signal (or a 64-point IDFT signal with the appropriatetones zeroed out) that includes OFDM symbols having the tone map 470 ofFIG. 7B (e.g., in the data, SIG, and/or LTF portions of the data units).In the embodiment and scenario shown in FIG. 21, the low bandwidth modesignal 1112 is transmitted in a frequency band that is fixed in asecond-lowest of four 0.5 MHz sub-bands within channel 1100-2. Moregenerally, in an embodiment, MAC layer operations (e.g., implemented byMAC processing unit 18 and/or MAC processing unit 28 of FIG. 1, invarious embodiments) do not permit the low bandwidth mode signal 1112 tobe transmitted in an upper-most or a lower-most sub-band of any ofcommunication channels 1100. In this manner, interference with otherchannels may generally be reduced.

In some of the embodiments described above with reference to FIGS.18-21, a low bandwidth mode signal with an unbalanced number of guardtones (i.e., more guard tones at the upper/lower band edge than thelower/upper band edge, as in the example tone map 450 of FIG. 7A) may betransmitted in a frequency band that places the smaller number of guardtones at one edge of the communication channel 1100-2. For example, withreference to the frequency band placement shown in FIG. 19, and in anembodiment where both low bandwidth mode signals 1106-1 and 1106-2 usethe tone map 450 of FIG. 7A, the low bandwidth mode signal 1106-2provides only two guard tones at the upper edge of channel 1100-2 (ascompared to the three guard tones that signal 1106-1 provides at thelower edge of channel 1100-2). As another example, if the low bandwidthmode signal 1110 in FIG. 20 were transmitted in a frequency band placedin the upper sideband of channel 1100-2 rather than the lower sideband(again, for the case where signal 1110 uses tone map 450 of FIG. 7A),the low bandwidth mode signal 1110 provides only two guard tones at theupper edge of channel 1100-2.

To increase the number of guard tones at the edge(s) of the channel1100-2, the tones of a low bandwidth mode signal (or of one or morefrequency domain duplicates thereof) are in some embodiments reversed orshifted. FIGS. 22A-22C are diagrams of example regular, reversed, andshifted tone maps 1150, 1160, and 1170 each corresponding to lowbandwidth mode signals, according to various embodiments. FIGS. 22A-22Ccorrespond to regular and re-routed (reversed or shifted) tone maps forthe case in which the regular tone map of low bandwidth mode signals isthe tone map 450 of FIG. 7A. Thus, the “regular” tone map 1150, withdata and pilot tones 1152, DC tone 1154, and guard tones 1158, isidentical to the tone map 450 of FIG. 7A with data and pilot tones 452,DC tone 454, and guard tones 458.

Tone map 1160 includes a same number (as compared to regular tone map1150) of data and pilot tones 1162, DC tone 1164, and guard tones 1168,but has two lower-edge guard tones 1168-1 and three upper edge-guardtones 1168-2 rather than vice versa. In this embodiment, the reversal ofthe number of guard tones at the band edges is achieved by reversing allnon-zero tones of the map 1150, i.e., such that the tones at indices −1to −13 in map 1150 are instead mapped to indices +1 to +13,respectively, in map 1160, and such that the tones at indices +1 to +13in map 1150 are instead mapped to indices −1 to −13, respectively, inmap 1160.

Tone map 1170 likewise includes a same number as compared to regulartone map 1150) of data and pilot tones 1172, DC tone 1174, and guardtones 1178, but has two lower-edge guard tones 1178-1 and three upperedge-guard tones 1178-2. In this embodiment, the reversal of the numberof guard tones is achieved by shifting all non-zero tones of the map1150 one to the left (except for the tone at the +1 index, which isshifted two to the left to avoid the DC tone). Thus, the tone at index+1 in map 1150 is instead mapped to index −1 in map 1170, the tones atindices −1 to −13 in map 1150 are instead mapped to indices −2 to −14,respectively, in map 1170, and the tones at indices +2 to +13 in map1150 are instead mapped to indices +1 to +12, respectively, in map 1170.

FIGS. 23A and 23B are diagrams of example regular and shifted tone maps1250 and 1260, respectively, each corresponding to low bandwidth modesignals, according to an embodiment. Unlike the regular and shifted tonemaps 1150 and 1170 of FIGS. 22A and 22C, however, the tone maps 1250 and1260 correspond to an embodiment in which the low bandwidth mode signalis transmitted in a frequency band fixed in the lower sideband of thenormal mode, 64-point IDFT signal channel (e.g., the embodiment of FIG.20). Thus, the tone indices shown correspond to the −32 to 0 indices ofthe lower sideband of the 64-point IDFT signal channel rather than the−15 to +16 indices or −16 to +15 indices of the 32-point IDFT signalfrequency band.

As with FIGS. 22A and 22C, the embodiments of FIGS. 23A and 23Bcorrespond to regular and shifted tone maps for the case in which theregular tone map of low bandwidth mode signals is the tone map 450 ofFIG. 7A. Thus, the “regular” tone map 1250, with data and pilot tones1252, DC tone 1254, and guard tones 1258, is identical to the tone map450 of FIG. 7A with data and pilot tones 452, DC tone 454, and guardtones 458, except that the map 1250 is aligned with the indices of thelower sideband of a 64-point IDFT signal channel.

Tone map 1260 includes a same number (as compared to regular tone map1250) of data and pilot tones 1262, DC tone 1264, and guard tones 1268,but has four lower-edge guard tones 1268-1 rather than three. In thisembodiment, the greater number of lower-edge guard tones is achieved byshifting all non-zero tones of the map 1250 one to the right, i.e., suchthat the tones at indices −3 to −15 in map 1250 are instead mapped toindices −2 to −15, respectively, in map 1260, and such that the tones atindices −17 to −29 in map 1250 are instead mapped to indices −16 to −28,respectively, in map 1260. In other embodiments, the tones of theregular tone map 1250 are instead shifted to the right by a differentsuitable number greater than one to provide even more lower-edge guardtones. In still other embodiments, for example where the low bandwidthmode signal is transmitted in a frequency band placed in the uppersideband of channel 1100-2 rather than the lower sideband, the tones areinstead shifted to the left by one or more indices.

In some embodiments, tone re-routing (reversing and/or shifting) of lowbandwidth mode signals (or one or more frequency-domain duplicatesthereof) is utilized as in FIGS. 22A-22C or as in FIGS. 23A and 23B, butthe tones of the STF portion of the low bandwidth mode signals (orduplicates) are unchanged, i.e., not reversed, shifted, or otherwisere-routed. In this manner, the periodicity of the STF sequences ispreserved.

FIG. 24 is a flow diagram of an example method 1400 for generating andcausing to be transmitted first and second data units conforming tofirst and second PHY modes, respectively, according to an embodiment. Inan embodiment, the first PHY mode is a normal mode of a long rangecommunication protocol and the second PHY mode is a low bandwidth modeof the long range communication protocol. For example, in oneembodiment, the second PHY mode is a control mode. Alternatively, thesecond PHY mode simply provides range extension beyond the first PHYmode. In some embodiments, the first PHY mode corresponds to a datathroughput that is greater than a data throughput corresponding to thesecond PHY mode. The method 1400 is implemented in a communicationsystem (e.g., WLAN 10 of FIG. 1) having a plurality of channels fortransmitting data units conforming to the first (e.g., normal) PHY mode.In some embodiments, the communication system also utilizes additional,composite channels (formed by aggregating two or more channels of theplurality of channels) to transmit data units conforming to the firstPHY mode. In an embodiment, the method 1400 is implemented by thenetwork interface 16 of AP 14 and/or the network interface 27 of clientstation 25-1 of FIG. 1.

At block 1402, the first data unit conforming the first PHY mode isgenerated, at least in part by generating a first series of OFDMsymbols. In one embodiment, the first series of OFDM symbols isgenerated at least in part by utilizing a 64-point IDFT. At block 1404,the first data unit generated at block 1402 is caused to be transmittedvia a first channel of the plurality of channels. For example, in anembodiment, a PHY processing unit within a network interfaceimplementing the method 1400 provides the OFDM signal corresponding tothe first data unit to a radio frequency (RF) transmit chain.

At block 1406, a second data unit conforming to the second PHY mode isgenerated, at least in part by generating a second series of OFDMsymbols. In one embodiment, at least a portion (e.g., a data portion, adata and SIG field portion, a data, LTF and SIG field portion, etc.) ofthe second series of OFDM symbols includes more upper-edge guard tonesthan lower-edge guard tones. In another embodiment, at least a portionof the second series of OFDM symbols includes more lower-edge guardtones than upper-edge guard tones. In one embodiment, the second seriesof OFDM symbols is generated at least in part by utilizing a 32-pointIDFT. In another embodiment, the second series of OFDM symbols isgenerated at least in part by utilizing a 64-point IDFT with at leastone half of the total number of generated tones set equal to zero. In anembodiment, the second series of OFDM symbols is generated using thesame clock rate used to generate the first series of OFDM symbols.

At block 1410, a frequency band for transmitting the second data unit isdetermined. The frequency band has a bandwidth equal to the bandwidth ofeach channel of the plurality of channels, divided by an integer ngreater than or equal to two. In one embodiment, the integer n is equalto two (e.g., each channel has a 2 MHz bandwidth, and the determinedfrequency band has a 1 MHz bandwidth). In an embodiment where theportion of the second series of OFDM symbols includes more upper-edgeguard tones than lower-edge guard tones, determining the frequency bandat block 1410 includes excluding a lowest sub-band of each of one ormore channels in the plurality of channels (e.g., in all channels of theplurality of channels). Alternatively, in an embodiment where theportion of the second series of OFDM symbols includes more lower-edgeguard tones than upper-edge guard tones, determining the frequency bandat block 1410 includes excluding a highest sub-band of each of one ormore channels in the plurality of channels (e.g., in all channels of theplurality of channels). In either case, each “sub-band” has a bandwidthequal to the bandwidth of the frequency band being determined at block1410 (i.e., the channel bandwidth divided by the integer n).Accordingly, the frequency band is placed within the channel such thatthe edge of the second PHY mode data unit having the lowest number ofguard tones is not aligned with a channel edge.

At block 1412, the second data unit is caused to be transmitted via thefrequency band determined at block 1410. For example, in an embodiment,a PHY processing unit within a network interface implementing the method1400 provides the OFDM signal corresponding to the second data unit toan RF transmit chain.

Whereas FIGS. 18-24 relate to channelization based on normal mode dataunit bandwidths, lower bandwidth regions (e.g., Europe, Japan, etc.) insome embodiments are channelized based on the low bandwidth mode dataunits. In these embodiments, the channel bandwidth equals the bandwidthof a low bandwidth mode data unit (e.g., 1 MHz), and normal mode dataunits (e.g., 2 MHz and greater) are transmitted in composite channelsformed by aggregating the narrower channels.

In one more specific example embodiment, a dual mode device (e.g.,client station 25-1 of FIG. 1), when used in a low bandwidth region, ischannelized based on a 1 MHz bandwidth corresponding to 32-point IDFTsignals of a low bandwidth mode PHY. In some such embodiments, 64-pointIDFT (2 MHz) normal mode data units using the same clock rate are alsopermitted in the low bandwidth region. 2 MHz channels are formed byaggregating two or more of the 1 MHz channels, with no overlap betweenthe various composite 2 MHz channels. In a 2 MHz basic service set (BSS)in such a region, the presence of a low bandwidth mode 1 MHz signalcould be in either the lower or upper sideband of the 2 MHz compositechannel, depending on which sideband corresponds to a 1 MHz primarychannel in a particular scenario.

In one such embodiment and scenario, 128-point, 256-point, and 512-pointIDFT signals (corresponding to 4 MHz, 8 MHz, and 16 MHz signals) aredisallowed, even if the dual mode device is configured to support thesewider-band normal mode signals in other regions. In other of theseembodiments, 128-point, 256-point, and/or 512-point IDFT signals areallowed. Moreover, in some embodiments, the low bandwidth mode signal isallowed more MCSs than in the wider bandwidth regions. A receiver inthis embodiment and scenario auto-detects whether a received data unitis a 32-point of 64-point IDFT (1 MHz or 2 MHz) signal (e.g., using anSTF, LTF, and/or SIG field) based on a priori knowledge of whether thelower or upper sideband of the composite 2 MHz channel corresponds theprimary 1 MHz channel. In embodiments where 128-point, 256-point, and/or512-point IDFT signals are allowed, auto-detection of which bandwidthsignal is received is based on a SIG field (e.g., an HTSIG field as inthe IEEE 802.11n and IEEE 802.11ac Standards). In an embodiment,32-point IDFT signals that correspond to a primary channel and arelocated in a sideband of the 2 MHz composite channel may utilize thetone re-routing techniques of FIG. 22 or 23 if needed to increase thenumber of guard bands at one or both edges of the 2 MHz compositechannel.

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. Likewise, the software or firmware instructionsmay be delivered to a user or a system via any known or desired deliverymethod including, for example, on a computer readable disk or othertransportable computer storage mechanism or via communication media.Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism. The term“modulated data signal” means a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia includes wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency,infrared and other wireless media. Thus, the software or firmwareinstructions may be delivered to a user or a system via a communicationchannel such as a telephone line, a DSL line, a cable television line, afiber optics line, a wireless communication channel, the Internet, etc.(which are viewed as being the same as or interchangeable with providingsuch software via a transportable storage medium). The software orfirmware instructions may include machine readable instructions that,when executed by the processor, cause the processor to perform variousacts.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), etc.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, changes, additions and/or deletions may bemade to the disclosed embodiments without departing from the scope ofthe claims.

1. A method, in a communication system, of generating and causing to be transmitted data units conforming to a first physical layer (PHY) mode and data units conforming to a second PHY mode different than the first PHY mode, wherein the communication system utilizes a plurality of channels for transmitting data units conforming to the first PHY mode, and wherein each channel of the plurality of channels has a first bandwidth, the method comprising: generating a first data unit conforming to the first PHY mode, wherein generating the first data unit includes generating a first series of orthogonal frequency division multiplexing (OFDM) symbols; causing the first data unit to be transmitted via a channel of the plurality of channels; generating a second data unit conforming to the second PHY mode, wherein generating the second data unit includes generating a second series of OFDM symbols, and at least a portion of the second series of OFDM symbols includes one of (i) more upper-edge guard tones than lower-edge guard tones, or (ii) more lower-edge guard tones than upper-edge guard tones: determining a frequency band for transmitting the second data unit, wherein the frequency band has a second bandwidth equal to the first bandwidth divided by an integer n, wherein n≧2, determining the frequency band for transmitting the second data unit includes excluding one of (i) a lowest sub-band of each of one or more channels in the plurality of channels when the portion of the second series of OFDM symbols includes more upper-edge guard tones than lower-edge guard tones or (ii) a highest sub-band of each of the one or more channels in the plurality of channels when the portion of the second series of OFDM symbols includes more lower-edge guard tones than upper-edge guard tones, and each sub-band of each channel in the plurality of channels has the second bandwidth; and causing the second data unit to be transmitted via the determined frequency band.
 2. A method according to claim 1, wherein the integer n is equal to
 2. 3. A method according to claim 2, wherein: generating the first series of OFDM symbols includes utilizing a 64-point inverse fast Fourier transform (IDFT); and generating the second series of OFDM symbols includes utilizing a 32-point IDFT.
 4. A method according to claim 2, wherein: generating the first series of OFDM symbols includes utilizing a 64-point inverse fast Fourier transform (IDFT); and generating the second series of OFDM symbols includes utilizing a 64-point IDFT h at least one half of a total number of generated tones set equal to zero.
 5. A method according to claim 1, wherein: generating the first series of OFDM symbols includes utilizing a first clock rate; and generating the second series of OFDM symbols includes utilizing the first clock rate.
 6. A method according to claim 1, wherein the first PHY mode corresponds to a first data throughput and the second PHY mode corresponds to a second data throughput lower than the first data throughput.
 7. A method according to claim 1, wherein the portion ofthc second series of OFDM symbols includes a data portion of the second series of OFDM symbols.
 8. A method according to claim 1, wherein: the portion of the second series of OFDM symbols includes more lower-edge guard tones than upper-edge guard tones; and determining the frequency band for transmitting the second data unit includes excluding the highest sub-band of the one or more channels in the plurality of channels.
 9. A method according to claim 1, wherein the one or more channels in the plurality of channels include all channels in the plurality of channels.
 10. An apparatus comprising: a network interface configured to generate a first data unit conforming to a first PHY mode at least in part by generating a first series of orthogonal frequency division multiplexing (OFDM) symbols, cause the first data unit to be transmitted via a channel of a plurality of channels each having a first bandwidth, generate a second data unit conforming to a second PHY mode different than the first PHY mode at least in part by generating a second series of OFDM symbols, wherein at least a portion of the second series of OFDM symbols includes one of (i) more upper-edge guard tones than lower-edge guard tones, or (ii) more lower-edge guard tones than upper-edge guard tones, determine a frequency band for transmitting the second data unit, wherein the frequency band has a second bandwidth equal to the first bandwidth divided by an integer n, wherein n≧2, the network interface is configured to determine the frequency band for transmitting the second data unit at least in part by excluding one of (i) a lowest sub-band of cach of one or more channels in the plurality of channels when the portion of the second series of OFDM symbols includes more upper-edge guard tones than lower-edge guard tones or (ii) a highest sub-band of each of the one or more channels in the plurality of channels when the portion of the second series of OFDM symbols includes more lower-edge guard tones than upper-edge guard tones, and each sub-band of each channel in the plurality of channels has the second bandwidth, and cause the second data unit to be transmitted via the determined frequency band.
 11. An apparatus according to claim 10, wherein the integer n is equal to
 2. 12. An apparatus according to claim 11, wherein the network interface is configured to generate the first series of OFDM symbols at least in part by utilizing a 64-point inverse fast Fourier transform (IDFT), and generate the second series of OFDM symbols at least in part by utilizing a 32-point IDFT.
 13. An apparatus according to claim 11, wherein the network interface is configured to generate the first series of OFDM symbols at least in part by utilizing a 64-point inverse fast Fourier transform (IDFT), and generate the second series of OFDM symbols at least in part by utilizing a 64-point IDFT with at least one half of a total number of generated tones set equal to zero.
 14. An apparatus according to claim 10, wherein the network interface is configured to generate the first series of OFDM symbols at leas n part by utilizing a first clock rate, and generate the second series of OFDM symbols at least in part by utilizing the first clock rate.
 15. An apparatus according to claim 10, wherein the first PHY mode corresponds to a first data throughput and the second PHY mode corresponds to a second data throughput lower than the first data throughput.
 16. An apparatus according to claim 10, wherein the portion of the second series of OFDM symbols includes a data portion of the second series of OFDM symbols.
 17. An apparatus according to claim 10, wherein: the portion of the second series of OFDM symbols includes more lower-edge guard tones than upper-edge guard tones, and the network interface is configured to determine the frequency band for transmitting the second data unit at least in part by excluding the highest sub-band of the one or more channels in the plurality of channels.
 18. An apparatus according to claim 10, wherein the one or more channels in the plurality of channels include all channels in the plurality of channels.
 19. A method, in a communication system, of generating and causing to be transmitted data units conforming to a first physical layer (PHY) mode and data units conforming to a second PHY mode different than the first PHY mode, wherein the communication system utilizes a plurality of channels for transmitting data units conforming to the first PHY mode, and wherein each channel of the plurality of channels has a first bandwidth, the method comprising: generating a first data unit conforming to the first PHY mode, wherein generating the first data unit includes generating a first series of orthogonal frequency division multiplexing (OFDM) symbols utilizing a clock rate; causing the first data unit to be transmitted via a channel of the plurality of channels; generating a second data unit conforming to the second PHY mode, wherein generating the second data unit includes generating a second series of OFDM symbols utilizing the clock rate, and at least a data portion of the second series of OFDM symbols includes more lower-edge guard tones than upper-edge guard tones; determining a frequency band for transmitting the second data unit, wherein the frequency band has a second bandwidth equal to half the first bandwidth, and determining the frequency band for transmitting the second data unit includes excluding an upper sideband of each of one or more channels in the plurality of channels; and causing the second data unit to be transmitted via the determined frequency band.
 20. An apparatus comprising: a network interface configured to generate a first data unit conforming to a first PHY mode at least in part by generating a first series of orthogonal frequency division multiplexing (OFDM) symbols utilizing a clock rate, cause the first data unit to be transmitted via a channel of a plurality of channels each having a first bandwidth, generate a second data unit conforming to a second PHY mode different than the first PHY mode at least in part by generating a second series of OFDM symbols utilizing the clock rate, wherein at least a data portion of the second series of OFDM symbols includes more lower-edge guard tones than upper-edge guard tones. determine a frequency band for transmitting the second data unit, wherein the frequency band has a second bandwidth equal to half the first bandwidth, and the network interface is configured to determine the frequency band for transmitting the second data unit at least in part by excluding an upper sideband of each of one or more channels in the plurality of channels, and cause the second data unit to be transmitted via the determined frequency band. 